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# Limit of detection of field effect transistor biosensors: Effects of surface modification and size dependence

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## Abstract

Field-effect transistor biosensors have shown great promise in the detection of biomolecules. However, a quantitative understanding of what limits the smallest measurable concentration of analyte (limit of detection or LOD) is still missing. By considering the signal-to-noise ratio (SNR), extracted from a combination of noise and I-V characterization, we are able to accurately predict and experimentally confirm a LOD of 0.01 pH. Our results also show that devices with larger area and with amine functionalized surfaces have larger SNR. We are able to extract the associated oxide trap densities and, thus, quantify the improvements in LOD.

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... improved macromolecular sensing can be achieved with smaller devices [30], no consensus has been reached on the effect of size reduction on pH sensitivity as well as the role of device technology, device layout, bias and signal readout [31,32]. In what follows, we show that tri-gate silicon nanoribbons with rectangular cross-section exhibit an enhanced sensitivity vs. pH when decreasing their width below a few hundreds of nanometers. ...
... Interestingly, this result is in contrast with other works [32], in which a linear dependence of the transconductancewith the factor (W NR + 2¨t NR )/L NR is found. The enhancement of the transconductance with smaller W NR could be a signature of marked corner effects, typical of devices with high body doping and rectangular cross-section [45], as opposed to low-doped trapezoidal wires [32]. ...
... Interestingly, this result is in contrast with other works [32], in which a linear dependence of the transconductancewith the factor (W NR + 2¨t NR )/L NR is found. The enhancement of the transconductance with smaller W NR could be a signature of marked corner effects, typical of devices with high body doping and rectangular cross-section [45], as opposed to low-doped trapezoidal wires [32]. ...
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The signal-to-noise ratio of planar ISFET pH sensors deteriorates when reducing the area occupied by the device, thus hampering the scalability of on-chip analytical systems which detect the DNA polymerase through pH measurements. Top-down nano-sized tri-gate transistors, such as silicon nanowires, are designed for high performance solid-state circuits thanks to their superior properties of voltage-to-current transduction, which can be advantageously exploited for pH sensing. A systematic study is carried out on rectangular-shaped nanowires developed in a complementary metal-oxide-semiconductor (CMOS)-compatible technology, showing that reducing the width of the devices below a few hundreds of nanometers leads to higher charge sensitivity. Moreover, devices composed of several wires in parallel further increase the exposed surface per unit footprint area, thus maximizing the signal-to-noise ratio. This technology allows a sub milli-pH unit resolution with a sensor footprint of about 1 µm², exceeding the performance of previously reported studies on silicon nanowires by two orders of magnitude.
... SNR is another important performance metric for sensors since it provides an estimate of the detection limit. [16][17][18] Analogous to the SNR analysis for nanowire FET sensors, 16 SNR for a bipolar transistor based sensor can also be written as ...
... SNR is another important performance metric for sensors since it provides an estimate of the detection limit. [16][17][18] Analogous to the SNR analysis for nanowire FET sensors, 16 SNR for a bipolar transistor based sensor can also be written as ...
... In Fig. 3(c), the measured SNR of the HBT and nanowire FET biosensors are compared. Since the SNR value depends on the device area (SNR / area À1/2 ), 16 it is important to compare these two types of sensors with similar areas. The nanowire FET sensor data are for devices $0.5 lm 2 from Ref. 17. ... Article Full-text available Important performance metrics, such as sensitivity and signal to noise ratio (SNR) of bipolar transistor based biosensors, are compared to those for nanowire field effect transistor (FET) sensors. The sensor consists of a heterojunction bipolar transistor (HBT) with silicon germanium base connected to a sensing surface in contact with the solution. The measured sensitivity is ≥2 times and SNR is >20 times higher in comparison to those for nanowire FET sensors. More importantly, the HBT biosensor sensitivity is constant over the sensing range of ∼5 decades and depends only on the temperature. In comparison, the nanowire FET sensor sensitivity varies in a complex manner over the sensing range and exhibits significant fabrication induced sensor to sensor variations. Consequently, HBT sensors would require minimal calibration for quantitative sensing studies. Furthermore, the bipolar transistor SNR is not only significantly higher but is also constant over the sensing range. In comparison, the nanowire FET sensor SNR varies with the peak value confined over a narrow sensing range. Hence, HBT sensor has <20 times lower detection limit that remains constant over the sensing range. In summary, HBT sensors are demonstrated to have superior performance metrics and are better suited for quantitative studies. Lastly, these HBT sensors also provide simultaneous temperature measurement. ... Empirically, there are various methods for determining the noise component. Given the low-frequency noise is of interest for biosensing, and that the most common noise source of FET-transistors is of the form 1/f (flicker noise), a common approach 46,52,107,129,130 utilises the following expression: ... ... Rajan et al. defined the SNR as the transconductance (g m ) divided by the square root of the current noise power density (S I ) and measured this in pH sensing experiments. 129,130 Focusing on the low-frequency range (around 1 Hz), they found that variation in ionic strength produced a negligible change in the SNR and concluded that the SNR an intrinsic property of the device. 130 They showed that the SNR can be maximised by tuning the gate voltages, 130 and increasing sensor area. ... ... 130 They showed that the SNR can be maximised by tuning the gate voltages, 130 and increasing sensor area. 129 They also stated that their device had a surface functionalised with (3-aminopropyl)triethoxysilane (APTES) and demonstrated improved SNR and reduced current noise power compared with a bare SiO 2 surface, 129 hypothesising that the APTES lowered the effective density of trapped charges at the oxide-electrolyte interface. ... Article Full-text available Field-Effect Transistor sensors (FET-sensors) have been receiving increasing attention for biomolecular sensing over the last two decades due to their potential for ultra-high sensitivity sensing, label-free operation, cost reduction and miniaturisation. Whilst the commercial application of FET-sensors in pH sensing has been realised, their commercial application in biomolecular sensing (termed BioFETs) is hindered by poor understanding of how to optimise device design for highly reproducible operation and high sensitivity. In part, these problems stem from the highly interdisciplinary nature of the problems encountered in this field, in which knowledge of biomolecular-binding kinetics, surface chemistry, electrical double layer physics and electrical engineering is required. In this work, a quantitative analysis and critical review has been performed comparing literature FET-sensor data for pH-sensing with data for sensing of biomolecular streptavidin binding to surface-bound biotin systems. The aim is to provide the first systematic, quantitative comparison of BioFET results for a single biomolecular analyte, specifically Streptavidin, which is the most commonly used model protein in biosensing experiments, and often used as an initial proof-of-concept for new biosensor designs. This novel quantitative and comparative analysis of the surface potential behaviour of a range of devices demonstrated a strong contrast between the trends observed in pH-sensing and those in biomolecule-sensing. Potential explanations are discussed in detail and surface-chemistry optimisation is shown to be a vital component in sensitivity-enhancement. Factors which can influence the response, yet which have not always been fully appreciated, are explored and practical suggestions are provided on how to improve experimental design. ... Rajan et al. pointed out that scaling did not always lead to devices with higher performance. A linear dependence was observed between SNR and √ WL, which encouraged the design of higher-surface-area EGFETs for improved sensitivity and limits of detection [44]. The design of an optimal gate area rather than gate width is preferred because flicker noise contribution depends on WL and is independent of W/L ratio and the DC bias conditions [45]. ... ... Van der Spiegel et al. proposed an extended-gate FET with an aspect ratio of 1900/10 µm/µm with the aim of improving gm and reducing the noise [14]. SNR is bias-dependent and thus can be optimized in order to further reduce power consumption [44]. ... ... Rajan et al. pointed out that scaling did not always lead to devices with higher performance. A linear dependence was observed between SNR and √WL, which encouraged the design of higher-surfacearea EGFETs for improved sensitivity and limits of detection [44]. The design of an optimal gate area ... Preprint Full-text available Since 1970s, a great deal of attention has been paid to the development of semiconductor–based biosensors because of the numerous advantages they offer, including high sensitivity, faster response time, miniaturization, and low–cost manufacturing for quick biospecific analysis with reusable features. Commercial biosensors have become highly desirable in the fields of medicine, food, environmental monitoring as well as military applications (e.g., Hoffmann–La Roche, Abbott Point of Care, Orion High technologies, etc.), whereas increasing concerns on the food safety and health issues have resulted in the introduction of novel legislative standards for these sensors. Numerous devices have been developed for monitoring of biological–processes such as nucleic–acid hybridization, protein–protein interaction, antigen–antibody bonds and substrate–enzyme reactions, just to name a few. Since 1980s scientific interest moved to the development of semiconductor–based devices which also include integrated front–end electronics, such as the extended–gate–field–effect–transistor biosensor which is one of the first miniaturized chemical sensors. This work is intended to be a review of the state of the art focused on the development of biosensors based extended–gate–field–effect–transistor within the field of bioanalytical applications, which will highlight the most recent research works reported in the literature. Moreover, a comparison among the diverse EGFET devices will be presented giving particular attention to the materials and technologies. ... Rajan et al. pointed out that scaling did not always lead to devices with higher performance. A linear dependence was observed between SNR and WL, which encouraged the design of higher-surface-area EGFETs for improved sensitivity and limits of detection [44]. The design of an optimal gate area rather than gate width is preferred because flicker noise contribution depends on WL and is independent of W/L ratio and the DC bias conditions [45]. ... ... Van der Spiegel et al. proposed an extended-gate FET with an aspect ratio of 1900/10 μm/μm with the aim of improving gm and reducing the noise [14]. SNR is bias-dependent and thus can be optimized in order to further reduce power consumption [44]. ... Article Full-text available Since the 1970s, a great deal of attention has been paid to the development of semiconductor-based biosensors because of the numerous advantages they offer, including high sensitivity, faster response time, miniaturization, and low-cost manufacturing for quick biospecific analysis with reusable features. Commercial biosensors have become highly desirable in the fields of medicine, food, and environmental monitoring as well as military applications, whereas increasing concerns about food safety and health issues have resulted in the introduction of novel legislative standards for these sensors. Numerous devices have been developed for monitoring biological processes such as nucleic acid hybridization, protein–protein interaction, antigen–antibody bonds, and substrate–enzyme reactions, just to name a few. Since the 1980s, scientific interest moved to the development of semiconductor-based devices, which also include integrated front-end electronics, such as the extended-gate field-effect transistor (EGFET) biosensor, one of the first miniaturized chemical sensors. This work is intended to be a review of the state of the art focused on the development of biosensors and chemosensors based on extended-gate field-effect transistor within the field of bioanalytical applications, which will highlight the most recent research reported in the literature. Moreover, a comparison among the diverse EGFET devices will be presented, giving particular attention to the materials and technologies. ... In the past, there have been significant efforts on improving the carrier mobility in the OSC channels through material molecule design and crystallizationcontrolled processing methods 12,13 . However, for biochemical sensors to detect very low concentration of analyte in various portable, wearable, or implantable scenarios, large SNR with low operating voltage and power would be a prerequisite [14][15][16] . However, there is lack of studies on optimal design of the OFET for such figure of merits considering the interplay between device structures and material stacks under processing constraints. ... ... The detection limit of the whole system is determined by the signal-to-noise ratio (SNR) of the OFET transducer, which is given as 16,20 SNR ¼ 10 log P signal P noise (2) where P signal is the signal power and P noise is the noise power. ... Article Full-text available Developing organic field-effect transistor (OFET) biosensors for customizable detection of biomarkers for many diseases would provide a low-cost and convenient tool for both biological studies and clinical diagnosis. In this work, design principles of the OFET transducer for biosensors were derived to relate the signal-to-noise ratio (SNR) to the device-performance parameters. Steep subthreshold swing (SS), proper threshold voltage (Vth), good-enough bias-stress stability, and mechanical durability are shown to be the key prerequisites for realizing OFET bio-sensors of high transconductance efficiency (gm/ID) for large SNR. Combining a low trap-density channel and a high-k/low-k gate dielectric layer, low-temperature (<100 °C) solution-processed flexible OFETs can meet the performance requirements to maximize the gm/ID. An extended gate-structure OFET biosensor was further implemented for label-free detection of miR-21, achieving a detection limit below 10 pM with high selectivity at a low operation voltage (<1 V). ... The minimum molecular concentration in the reception space required for the receiver to detect the existence of an information molecule is termed the limit of detection (LoD) [40]. It is not to be confused with the minimum concentration required for the exact determination of the concentration. ... ... There are only a few attempts to analytically model a biosensor, most of which are cited in this paper [32] [37] [40]. However, these works together with our model assume that the ligands to be detected are in equilibrium with the receptors at the time of detection. ... Article Molecular communications, where molecules are used to encode, transmit, and receive information, is a promising means of enabling the coordination of nanoscale devices. The paradigm has been extensively studied from various aspects, including channel modeling and noise analysis. Comparatively little attention has been given to the physical design of molecular receiver and transmitter, envisioning biological synthetic cells with intrinsic molecular reception and transmission abilities as the future nanomachines. However, this assumption leads to a discrepancy between the envisaged applications requiring complex communication interfaces and protocols, and the very limited computational capacities of the envisioned biological nanomachines. In this paper, we examine the feasibility of designing a molecular receiver, in a physical domain other than synthetic biology, meeting the basic requirements of nanonetwork applications. We first review the state-of-the-art biosensing approaches to determine whether they can inspire a receiver design. We reveal that nanoscale field effect transistor (FET)-based electrical biosensor is a particularly useful starting point towards designing a molecular receiver. Focusing on FET-based molecular receivers with a conceptual approach, we provide a guideline elaborating on their operation principles, performance metrics, and design parameters. We then develop a deterministic model for signal flow in silicon nanowire (SiNW) FET-based molecular receiver, and investigate its sensitivity for varying system parameters. Lastly, we discuss the practical challenges of implementing the receiver, and present the future research avenues from a communication theoretical perspective. ... Since the microenvironment of conductive material determines the features of the channel, any change at its surface alters the charge passing density, i.e., the current and the associated readout electrical metrics, a phenomenon termed field effect, which affects just the more near surfaces, decreasing rapidly with distance [77]. The last is particularly true for vdW materials like graphene and other lowdimensional structures such as nanowires and nanoribbons because their surface-to-volume ratio enhances the sensitivity and allows a very low limit of detection (LOD) [123,124]. As proof, increasing V GS decreases channel resistance to establish a significatively I DS because the basal V DS forces the charges to flow through in the S → D direction. ... ... Since most sensors require a minimum analyte threshold at their surface to produce a substantial output, the accumulation is also decisive. Before moving on, it is precise to assume that all the sensing region (i.e., G, channel, or WE) is covered by the bioreceptor since, otherwise, the signalto-noise ratio decreases while the signal derives to a more significant area than that where the analyte binding takes place [123]. ... Article Full-text available Smart electronic devices based on micro-controllers, also referred to as fashion electronics, have raised wearable technology. These devices may process physiological information to facilitate the wearer's immediate biofeedback in close contact with the body surface. Standard market wearable devices detect observable features as gestures or skin conductivity. In contrast, the technology based on electrochemical biosensors requires a biomarker in close contact with both a biorecognition element and an electrode surface, where electron transfer phenomena occur. The noninvasiveness is pivotal for wearable technology; thus, one of the most common target tissues for real-time monitoring is the skin. Noninvasive biosensors formats may not be available for all analytes, such as several proteins and hormones, especially when devices are installed cutaneously to measure in the sweat. Processes like cutaneous transcytosis, the paracellular cell–cell unions, or even reuptake highly regulate the solutes content of the sweat. This review discusses recent advances on wearable devices based on electrochemical biosensors for biomarkers with a complex blood-to-sweat partition like proteins and some hormones, considering the commented release regulation mechanisms to the sweat. It highlights the challenges of wearable epidermal biosensors (WEBs) design and the possible solutions. Finally, it charts the path of future developments in the WEBs arena in converging/emerging digital technologies. Graphical abstract ... where λ is the tunneling parameter for electrons in silicon oxide (~10 −10 m), N ot is the oxide trap density and f is the frequency at which the power of the noise density is measured. Thus, the SNR of an immunoFET is given by (Rajan et al., 2014) ... ... The effect of surface functionalization on device noise and perfor- mance have also been studied ( Delker et al., 2013;). Rajan et al. investigated the effect of APTES functionalization on the noise properties of bioFETs and reported a 3-fold increase in SNR which indicate almost an order of magnitude decrease in the trap density ( Rajan et al., 2014). Also, the SNR is directly proportional to area which means increasing the device area results in higher SNR. ... ... Here, silver and silver chlorinated electrodes have shown a very good performance [59]. Experimental studies by Rajan et al. have demonstrated a constant chloride concentration within the electrolyte, when using a silver chlorinated electrode [60]. This effect is explained by the strong interaction with the chloride within the electrolyte, indicating that this type of pseudo-reference electrode is suitable for biosensing. ... ... The advantage of nanowires is their high surface area to volume ratio, which allows small local charge changes at the transistor surface having a significant effect on the FET channel carrier concentration with a correlating change of the drain output current. Experimental developments in nanoscale BioFETs with improved response output signals by increasing the surface area to volume ratio have been published [64], [70], [62], [60], but theoretical explanations are still under investigation [71], [72]. The commonly used transfer curve of a FET is measured as a function of the gate voltage over the output drain current before and after an analyte addition (figure 2.5 a)). ... Thesis Full-text available p class="MsoNormal">Sensing of bacteria, viruses and biomolecules is increasingly important for environmental monitoring and healthcare applications. Electronic devices as transducer elements, that are scaled down to nanometre dimensions offer a sensitive electrical detection of bioanalytes. In this work, nanowire field effect transistors (NWFET) made from zinc oxide (ZnO) offer high sensitivity and low thermal budget fabrication compared to silicon nanowire sensors. The novelty of this work is a new low temperature top-down fabrication process, which makes it possible to define ZnO NWFET arrays with different numbers of nanowires simultaneously and systematically compare their electrical performance. The main feature of this process is a developed bilayer photoresist pattern with a retrograde profile, which enables the modification of the nanowire in width, length, height and the number of transistor channels. The approach is compatible with low cost manufacture without electron beam lithography and benefits from process temperatures below 150ºC. Process reliability has been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray diffraction (XRD) and atomic force microscopy (AFM). The nanowires exhibit a cross section dimension of 30.9 nm height and 257.4 nm width and varying lengths from 5 µm to 45 µm and show a very smooth top surface with a root mean squared (rms) roughness of only 1.2 nm. Electrical measurements demonstrate enhancement mode transistors, which show a scalable correlation between the number of nanowires and the electrical characteristics. Thereby, devices with 100 nanowires exhibit the best performance with a high field effect mobility of 11.0 cm<sup>2</sup>/Vs, on/off current ratio of 4 × 10<sup>7</sup> and subthreshold swing of 660 mV/dec. The fabricated multichannel ZnO NWFET have been investigated for their potential bio-sensing capabilities. The ZnO NWFET passivated with Al<sub>2</sub>O<sub>3</sub> is able to operate 16 hours continuously in phosphate buffered saline (PBS) solution with a very small current drift of 1.3 % per hour. It was found, that an Al<sub>2</sub>O<sub>3</sub> passivation layer of 30 nm gives the best electrical performance of the ZnO NWFETs. Hereby, the ZnO NWFET shows a very good recovery behaviour up to 81.8 % of its original signal output current. The output current of the ZnO NWFET shifts to different ionic strengths in aqueous solutions and changes during exposure with 10x, 100x and 1000x diluted PBS of up to 23.7%. Investigations on the sensing capabilities on proteins show that the ZnO NWFET responds at a very low drain voltage of 5 mV to varying charges within liquid solutions containing lysozyme and bovines serum albumin (BSA). An output current signal change between these two proteins of 295.5 % was measured, indicating a very good sensitivity of the ZnO nanowire channel to the presence of surrounding charges. After evidence was provided that the fabricated ZnO NWFET are capable for bio-sensing experiments, a mask design was developed, which allows to package the ZnO NWFET with gold wire bonding onto a polychlorinated biphenyl (PCB) board to enable statistical bio-sensing experiments. Hereby individual ZnO NWFETs can be addressed and measured by flushing laser cut poly(methyl methacrylate) (PMMA) micro fluidic channels with analytes solutions.</p ... Using the calibration data from the continuous time measurement of Figure 2 and assuming that small pH changes may be considered to result in a linear change of the biosensor current [23], the endpoint of the 25 µM urea substrate is equivalent to a change of 27 mpH. This result shows that our embedded system approaches the theoretical limit of 10 mpH for nanoribbon biosensors, as demonstrated in [25,26], without resorting to any additional filtering or complex read-out equipment. It also verifies the statement in [25] where a large area sensor is predicted to have a better LoD than an aggressively scaled device (i.e., nanoribbon vs nanowire sensor). ... ... This result shows that our embedded system approaches the theoretical limit of 10 mpH for nanoribbon biosensors, as demonstrated in [25,26], without resorting to any additional filtering or complex read-out equipment. It also verifies the statement in [25] where a large area sensor is predicted to have a better LoD than an aggressively scaled device (i.e., nanoribbon vs nanowire sensor). Regarding the noise properties of the proposed system, as can be seen from Figure 3 and particularly in Figure 4, the noise level is low enough to enable urea measurements to concentrations down to 25 µM. ... Article Full-text available We present a complete biosensing system that comprises a Thin Film Transistor (TFT)-based nanoribbon biosensor and a low noise, high-performance bioinstrumentation platform, capable of detecting sub-30 mpH unit changes, validated by an enzymatic biochemical reaction. The nanoribbon biosensor was fabricated top-down with an ultra-thin (15 nm) polysilicon semiconducting channel that offers excellent sensitivity to surface potential changes. The sensor is coupled to an integrated circuit (IC), which combines dual switched-capacitor integrators with high precision analog-to-digital converters (ADCs). Throughout this work, we employed both conventional pH buffer measurements as well as urea-urease enzymatic reactions for benchmarking the overall performance of the system. The measured results from the urea-urease reaction demonstrate that the system can detect urea in concentrations as low as 25 µM, which translates to a change of 27 mpH, according to our initial pH characterisation measurements. The attained accuracy and resolution of our system as well as its low-cost manufacturability, high processing speed and portability make it a competitive solution for applications requiring rapid and accurate results at remote locations; a necessity for Point-of-Care (POC) diagnostic platforms. ... Signal-to-Noise ratio (SNR) is an important factor that together with the sensitivity, defines the limit of detection for biosensors [32,33]. SNR was reported to be mainly affected by the intrinsic device quality [32] and to increase with the square root of channel area [33]. ... ... Signal-to-Noise ratio (SNR) is an important factor that together with the sensitivity, defines the limit of detection for biosensors [32,33]. SNR was reported to be mainly affected by the intrinsic device quality [32] and to increase with the square root of channel area [33]. In our work, it has been shown that aggressive scaling is not necessary to achieve high sensitivity and therefore devices such as nanoribbons with large channel areas can be used to deliver high sensitivity without compromising the SNR. ... Preprint In this work, we investigate how the sensitivity of a nanowire or nanoribbon sensor is influenced by the subthreshold slope of the sensing transistor. Polysilicon nanoribbon sensors are fabricated with a wide range of subthreshold slopes and the sensitivity is characterized using pH measurements. It is shown that there is a strong relationship between the sensitivity and the device subthreshold slope. The sensitivity is characterized using the current sensitivity per pH, which is shown to increase from 1.2%/pH to 33.6%/pH as the subthreshold slope improves from 6.2 V/dec to 0.23 V/dec respectively. We propose a model that relates current sensitivity per pH to the subthreshold slope of the sensing transistor. The model shows that sensitivity is determined only on the subthreshold slope of the sensing transistor and the choice of gate insulator. The model fully explains the values of current sensitivity per pH for the broad range of subthreshold slopes obtained in our fabricated nanoribbon devices. It is also able to explain values of sensitivity reported in the literature, which range from 2.5%/pH to 650%/pH for a variety of nanoribbon and nanowire sensors. Furthermore, it shows that aggressive device scaling is not the key to high sensitivity. For the first time, a figure-of-merit is proposed to compare the performance of nanoscale field effect transistor sensors fabricated using different materials and technologies. ... The signal-to-noise ratio (SNR), which is defined as g S / m ID , is used as a figure-ofmerit to examine the smallest value of analyte concentration that a sensor can detect. 41 Figure 5c shows the extracted S VG and SNR as a function of V GS at 10 Hz. The voltage noise S VG first decreases at a voltage ranging from −2.8 to −1 V and then increases gradually from −1 to 2.0 V. ... Article A stack consisting of atomic layer deposited (ALD) Aluminium Oxide (Al2O3) and Hexagonal Boron Nitride (h-BN) is proposed and demonstrated as the sensing layer of Extended Gate Ion-Sensitive Field-Effect Transistors (EG ISFETs) in this work. The use of h-BN in the sensing stack is to suppress the voltage drift during pH measurement, owing to a low defect level inside the two-dimensional (2D) h-BN. The use of Al2O3 in the sensing stack is to improve the pH sensitivity due to the chemical inertness of h-BN and the resultant low sensitivity. A low power oxygen plasma treatment on the surface of the h-BN flake is employed to obtain a uniform and high-quality Al2O3 layer on top of h-BN. The influences of Al2O3 thickness in the Al2O3/h-BN sensing stack on the sensitivity and drift characteristics of 2D EG ISFETs are investigated. It is found that the EG ISFET with Molybdenum Disulfide (MoS2) Field-Effect Transistor (FET) and Al2O3/h-BN sensing stack with 5 nm thick Al2O3 provides a pH sensitivity of higher than 50 mV/pH while at the same time exhibiting a low drift value of ~4 mV/hour. Moreover, a pH sensing resolution of 1.54 x 10⁻³ pH is extracted by using low frequency noise measurement. All these suggest the Al2O3/h-BN stack as a highly promising sensing stack for high sensitivity and low drift ISFETs. ... The phenomenon can be described as follows, when the analyte is adsorbed by the surface, a concentration gradient forms into the solution and the analytes further from the sensor must travel through it to reach the binding sites on the sensor surface. The steady-state signal is provided after the equilibrium is reached, and the time needed to reach this depends on sensor geometry (Nair and Alam 2006;Rajan et al. 2014). Nanowires provide faster response since they can sense molecules coming from the two dimensions perpendicular to the sensor surface, while planar FET can only collect molecules diffusing in one direction as an effect of the reduced dimensionality in the diffusion. ... Article We present a review of field effect transistors (FET) from the point of view of their applications to label-free sensing in the era of genomics and proteomics. Here, rather than a collection of Bio-FET achievements, we propose an analysis of the different issues hampering the use of these devices into clinical applications. We make a particular emphasis on the influence of the sensor geometry in the phenomena of mass transport of analytes, which is a topic that has been traditionally overlooked in the analysis and design of biosensors, but that plays a central role in the achievement of low limits of detection. Other issues like the screening of charges by the ions in liquids with physiological ionic strength and the non-specific binding are also reviewed. In conclusion, we give an overview of different solutions that have been proposed to address all these challenges, demonstrating the potential of field effect transistors owing to their ease of integration with other semiconductor components for developing cost-effective, highly multiplexed sensors for next-generation medicines. ... Signal to noise ratio (SNR) is another important sensor performance metric. As discussed elsewhere 26 , SNR measures the sensor resolution (i.e. smallest measurable change in ion concentration) and can be written as: Using equation 5, the SNR is estimated as follows. ... Article Full-text available Field effect transistors (FET) have been widely used as transducers in electrochemical sensors for over 40 years. In this report, a FET transducer is compared with the recently proposed bipolar junction transistor (BJT) transducer. Measurements are performed on two chloride electrochemical sensors that are identical in all details except for the transducer device type. Comparative measurements show that the transducer choice significantly impacts the electrochemical sensor characteristics. Signal to noise ratio is 20 to 2 times greater for the BJT sensor. Sensitivity is also enhanced: BJT sensing signal changes by 10 times per pCl, whereas the FET signal changes by 8 or less times. Also, sensor calibration curves are impacted by the transducer choice. Unlike a FET sensor, the calibration curve of the BJT sensor is independent of applied voltages. Hence, a BJT sensor can make quantitative sensing measurements with minimal calibration requirements, an important characteristic for mobile sensing applications. As a demonstration for mobile applications, these BJT sensors are further investigated by measuring chloride levels in artificial human sweat for potential cystic fibrosis diagnostic use. In summary, the BJT device is demonstrated to be a superior transducer in comparison to a FET in an electrochemical sensor. ... [70] In the case of silicon FET, the functionalization of the sensor channel (in this case a silicon nanowire buried in a SiO 2 dielectric) with 3-aminopropyl-triethoxysilane (APTES) yields better noise performances (up to 60 times), presumably REVIEW due to the passivation of the oxide traps and interface states at the sensor surface. [71] On the contrary, for carbon nanotubes, a two-level random telegraphic noise (RTN) was reported and ascribed to a single probe molecule (more precisely, the binding and unbinding of charged target biomolecules at its active sites), which was covalently bound to a defect in the carbon nanotube sidewall. [72] A suppression of the RTN was observed in high ionic strength buffer solutions (ionic screening) and for high gate potentials (when the target biomolecules are repelled from the nanotube). ... Article Full-text available Recent research trends now offer new opportunities for developing the next generations of label-free biochemical sensors using graphene and other two-dimensional materials. While the physics of graphene transistors operated in electrolyte is well grounded, important chemical challenges still remain to be addressed, namely the impact of the chemical functionalizations of graphene on the key electrical parameters and the sensing performances. In fact, graphene - at least ideal graphene - is highly chemically inert. The functionalizations and chemical alterations of the graphene surface - both covalently and non-covalently - are crucial steps that define the sensitivity of graphene. The presence, reactivity, adsorption of gas and ions, proteins, DNA, cells and tissues on graphene have been successfully monitored with graphene. This review aims to unify most of the work done so far on biochemical sensing at the surface of a (chemically functionalized) graphene field-effect transistor and the challenges that lie ahead. The authors are convinced that graphene biochemical sensors hold great promise to meet the ever-increasing demand for sensitivity, especially looking at the recent progresses suggesting that the obstacle of Debye screening can be overcome. ... However, for sub-femtomolar detection of target molecules, it is not sufficient to only explore the steady state sensitivities but is also necessary to investigate the flicker noise which dominates the noise spectrum at frequencies below 100 kHz. This low frequency noise in liquid gated condition which is dependent on the concentration of impurities at the surface and exposure time to solution [11,12] is an important parameter for determining the sensor resolution [13,14]. ... Article Full-text available Recently, a reproducible and scalable chemical method for fabrication of smooth graphene nanogrids has been reported which addresses the challenges of graphene nanoribbons (GNR). These nanogrids have been found to be capable of attomolar detection of biomolecules in field effect transistor (FET) mode. However, for detection of sub-femtomolar concentrations of target molecule in complex mixtures with reasonable accuracy, it is not sufficient to only explore the steady state sensitivities, but is also necessary to investigate the flicker noise which dominates at frequencies below 100 kHz. This low frequency noise is dependent on the exposure time of the graphene layer in the buffer solution and concentration of charged impurities at the surface. In this paper, the functionalization strategy of graphene nanogrids has been optimized with respect to concentration and incubation time of the cross linker for an enhancement in signal to noise ratio (SNR). It has been interestingly observed that as the sensitivity and noise power change at different rates with the functionalization parameters, SNR does not vary monotonically but is maximum corresponding to a particular parameter. The optimized parameter has improved the SNR by 50% which has enabled a detection of 0.05 fM Hep-B virus molecules with a sensitivity of around 30% and a standard deviation within 3%. Further, the SNR enhancement has resulted in improvement of quantification accuracy by five times and selectivity by two orders of magnitude. ... The non-linearity of the site binding response shows up as an increased S V for increasing pH, consistently with previously reported data for SiO 2 gate dielectric [14]. In addition, we note that the response for these low doped devices is essentially independent of W (compare graphs on the left and the right), consistently with recent literature on ISFETs with low channel doping and large device sizes [5,19]. Fig. 6 reports the differential current sensitivity (S I =∆I D [I D ∆pH], at a fixed V F G ) for the same devices of Fig. 5. Differently from the threshold voltage sensitivity, the current sensitivity exhibits a clear non-monotonic trend for increasing pH. ... Conference Paper We report accurate characterization, modelling and simulation of SOI nanoribbon-based pH sensors and we compare operation in air (dry) and electrolyte (wet) environments. We find remarkably different current density distributions and geometry scaling rules, but similar series resistances and active trap state densities in the two configurations. Calibrated TCAD based simulations implementing an original approach to model the site-binding charge, and in good agreement with experiments, provide the necessary insights to interpret the non trivial dependence of the threshold voltage and current sensitivity on pH. ... The continuous scaling of planar metal-oxide-semiconductor fieldeffect transistor (MOSFET) evidenced come technological difficulties whereby the device can no longer be classified as a long channel MOSFET. The main short-channel effect is due to the twodimensional distribution of potential and high electric fields in the channel region, which mainly lead to variable threshold voltage, saturation region that does not depend on drain potential, and drain current that does not depend on the inverse of channel length [12]. Subsequently a variety of different planar and non-planar topologies were investigated, such as the FinFET and the TFET [13,14]. ... Article Full-text available A long-standing goal of nanoelectronics is the development of integrated systems to be used in medicine as sensor, therapeutic, or theranostic devices. In this review, we examine the phenomena of transport and the interaction between electro-active charges and the material at the nanoscale. We then demonstrate how these mechanisms can be exploited to design and fabricate devices for applications in biomedicine and bioengineering. Specifically, we present and discuss electrochemical devices based on the interaction between ions and conductive polymers, such as organic electrochemical transistors (OFETs), electrolyte gated field-effect transistors (FETs), fin field-effect transistor (FinFETs), tunnelling field-effect transistors (TFETs), electrochemical lab-on-chips (LOCs). For these systems, we comment on their use in medicine. ... Generally, the vdW heterojunction BJT devices have higher sensitivity and detection capability than nanowires and graphene-based FET sensors. [14,15] This feature of the vdW heterojunction BJT devices may be potentially used in gas sensing, photodetection, and biosensing applications. [12,16,17] Here, we introduce an n-p-n BJT device composed of heavily doped molybdenum ditelluride (n-MoTe 2 ) and germanium selenide (p-GeSe) sheets stacked on each other by vdW interactions. ... Article Full-text available Bipolar junction transistors (BJTs), the basic building blocks of integrated circuits, are deployed to control switching applications and logic operations. However, as the thickness of a conventional BJT device approaches a few atoms, its performance decreases substantially. The stacking of atomically thin 2D semiconductor materials is advantageous for manufacturing atomically thin BJT devices owing to the high carrier density of electrons and holes. Here, an atomically thin n–p–n BJT device composed of heavily doped molybdenum ditelluride (n‐MoTe2) and germanium selenide (p‐GeSe) sheets stacked over each other by van der Waals interactions is reported. In a common‐emitter configuration, MoTe2/GeSe/MoTe2 BJT devices exhibit a considerably high current gain (β = Ic /Ib = 29.3) at Vbe = 2.5 V. The MoTe2/GeSe/MoTe2 BJT device is employed to detect streptavidin biomolecules as analytes within <10 s. The real‐time response of the functionalized BJT device is examined at various concentrations of streptavidin biomolecules ranging from 250 to 5 pm. Such vdW BJT devices can trigger the development of state‐of‐the‐art electronic devices that can be used as biosensors to detect the various kinds of target DNA and proteins like spike protein of Covid‐19. ... However, one of the important points that have to be addressed in this regard is the electrical noise of the transducers. It has traditionally been considered that electrical noise is the limiting factor affecting biosensor performance and sensitivity (Bedner et al., 2014;Chen et al., 2015;Cl� ement et al., 2011Cl� ement et al., , 2010Rajan et al., 2014). At the same time, the ultimate scaling down of the sizes of nanowires brings about new effects such as single-trap phenomena that can be used in order to enhance the sensitivity and efficiency of nanoscale biosensors (Gasparyan et al., 2015;Kutovyi et al., 2018b;Li et al., 2014;Pud et al., 2014). ... Article New highly sensitive direct methods for the early detection of peptides involved in Alzheimer's disease (AD) are required in order to prolong effective and healthy memory and thinking capabilities and also to stop the factors resulting in AD. In this contribution, we report the successful demonstration of a label-free approach for the detection of amyloid-beta (Aβ) peptides by highly selective aptamers immobilized onto the SiO2 surface of the fabricated sensors. A modified single-stranded deoxyribonucleic acid (ssDNA) aptamer was specially designed and synthesized to detect the target amyloid beta-40 sequence (Aβ-40). Electrolyte–insulator–semiconductor (EIS) structures as well as silicon (Si) nanowire (NW) field-effect transistors (FETs) covered with a thin SiO2 dielectric layer have been successfully functionalized with Aβ-40-specific aptamers and used to detect ultra-low concentrations of the target peptide. The binding of amyloid-beta peptides of different concentrations to the surface of the sensors varied in the range from 0.1 pg/ml to 10 μg/ml resulting in a change of the surface potential was registered by the fabricated devices. Moreover, we show that the single-trap phenomena observed in the novel Si two-layer (TL) NW FET structures with advanced characteristic parameters can be effectively used to increase the sensitivity of nanoscale sensors. The obtained experimental data demonstrate a highly sensitive and reliable detection of ultra-low concentrations of the Aβ-40 peptides. This opens up prospects for the development of real-time electrical biosensors for studying and understanding different stages of AD by utilizing Si TL NW FET structures fabricated on the basis of cost-efficient CMOS-compatible technology. ... Since the size of the biomolecules differs and could not cover the whole surface area of the nanocavity, for the detection of target biomolecules in the presence of parasitic biomolecules on the surface, this behaves like the noise. In the FET-based biosensor, the channel noise or disturbances are more prominent [28] due to the modification of dielectric at the source/channel interface. Therefore, to address this issue, we have measured and compared the SNR of the conventional DM-FET and DM-TFET with the proposed BP-JLFET-based biosensor (see Fig. 14). ... Article In this article, we have focused on the concept of junction-free transistor to propose and simulate the ultrathin dielectric modulated (DM) bulk-planar junctionless field-effect transistor (BP-JLFET) as a biosensor device. The proposed device is incorporated with a label-free detection of neutral (proteins, enzymes, streptavidin, and APTES) and charged [deoxyribonucleic acid (DNA)] biomolecules in terms of dielectric constant (K) and charge densities ($\rho $). For the detection of the biomolecules, the nanocavity is formed by etching out the oxide underneath the gate dielectric at source end. The presence/absence of biomolecules has been detected by the factor of sensitivity with the immobilization of dielectric constant (K) and the charge density ($\rho $) in the formed nanocavity. Moreover, the comparative study of the BP-JLFET, the dielectric-modulated tunnel field-effect transistor (DM-TFET), and the conventional dielectric-modulated field-effect transistor (DM-FET)-based biosensor in terms of their drain current (${\text I}_{ds}$) and the sensitivity have been carried out. From the study, it can be depicted that the BP-JLFET has higher sensitivity to sense the biomolecules compared to both the DM-TFET and the conventional DM-FET. Furthermore, we have also analyzed the noise characteristics for the simulated structures to measure the ability of the proposed device for sensing the biomolecules in the presence of noise. ... The surface potential sensitivity of chemFET can be modulated by a few parameters, including the fabrication parameters (e.g., doping, size) [68,69], device operation [52,70], the interface material [51,71], electrolyte ionic strength [72], surface chemistry [73], etc. Doping and size have a significant impact on the charge sensitivity. The reduction in size gives nanoFETs very large surface-to-volume ratio and makes them extremely sensitive to the surface properties. ... Article Full-text available Surface potential and surface charge sensing techniques have attracted a wide range of research interest in recent decades. With the development and optimization of detection technologies, especially nanosensors, new mechanisms and techniques are emerging. This review discusses various surface potential sensing techniques, including Kelvin probe force microscopy and chemical field-effect transistor sensors for surface potential sensing, nanopore sensors for surface charge sensing, zeta potentiometer and optical detection technologies for zeta potential detection, for applications in material property, metal ion and molecule studies. The mechanisms and optimization methods for each method are discussed and summarized, with the aim of providing a comprehensive overview of different techniques and experimental guidance for applications in surface potential-based detection. ... Efforts are being made to utilize the unusual structures of the BioFET, such as the 'Cell-based BioFET' and the 'Beetle/chip FET', which utilizes cell attaching to the gate insulator and an insect antenna with an olfactory receptor for sensing, respectively [1]. In addition to the material aspect, there are also studies in terms of signal processing such as quantitatively analyzing signal-to-noise ratio (SNR) to improve BioFET's performance [50]. Along with the studies about various materials, processing and analysis methods, the development of BioFET is endless. ... Article Interest in biomolecular sensors for diagnosis of early diseases and prognosis of the diseases is increasing day by day. Among them, FET-based sensors are very useful in that of their versatile operating characteristics using various materials. Herein, after addressing the basic principles of BioFET, we conduct an overall review of BioFET on two of the main structural elements: transducing materials and probes. Transducing materials were classified into graphene, carbon nanotube, silicon, MOF, etc., and probes were classified into antibodies, enzymes, aptamers, etc.. The important elements in designing BioFETs, such as electrical properties of each material, Debye length, and fabrication process are introduced along with their respective structures and materials. After the review of each of these structures and characteristics, examples are discussed along with sensitivity, selectivity, and limit of detection. In addition to the operating aspects of the senser, novel processes, treatments, and materials that can be considered for various purposes are also introduced. Based on the understanding, an overview of diverse examples is given by dividing the applications of BioFET into three main types: antigen sensing, biomarker sensing, and drug effect monitoring. Focusing on these general reviews, we conclude how the future direction of development will move forward and what the main challenge is. ... Therefore, proper surface functionalization is imperative for optimizing the BRE immobilization, enhancing the sensitivity, preventing unwanted reactions and minimizing the noise [96]. Additionally, the type of material used to cover the sensor's surface to increase its biocompatibility and surface chemistry play a crucial role in improving the performance of the sensor [97]. As mentioned in the previous section, a wide range of substrates is utilized for this purpose including gold, nanowire (NWs), CNT, graphene, glycan, etc. [98]. ... Article Full-text available Field-effect transistor (FET) biosensors have been intensively researched toward label-free biomolecule sensing for different disease screening applications. High sensitivity, incredible miniaturization capability, promising extremely low minimum limit of detection (LoD) at the molecular level, integration with complementary metal oxide semiconductor (CMOS) technology and last but not least label-free operation were amongst the predominant motives for highlighting these sensors in the biosensor community. Although there are various diseases targeted by FET sensors for detection, infectious diseases are still the most demanding sector that needs higher precision in detection and integration for the realization of the diagnosis at the point of care (PoC). The COVID-19 pandemic, nevertheless, was an example of the escalated situation in terms of worldwide desperate need for fast, specific and reliable home test PoC devices for the timely screening of huge numbers of people to restrict the disease from further spread. This need spawned a wave of innovative approaches for early detection of COVID-19 antibodies in human swab or blood amongst which the FET biosensing gained much more attention due to their extraordinary LoD down to femtomolar (fM) with the comparatively faster response time. As the FET sensors are promising novel PoC devices with application in early diagnosis of various diseases and especially infectious diseases, in this research, we have reviewed the recent progress on developing FET sensors for infectious diseases diagnosis accompanied with a thorough discussion on the structure of Chem/BioFET sensors and the readout circuitry for output signal processing. This approach would help engineers and biologists to gain enough knowledge to initiate their design for accelerated innovations in response to the need for more efficient management of infectious diseases like COVID-19. ... Biosensor has the two performance parameters namely detection limit and dynamic range. Limit of detection (LOD) is measured by using FET biosensors to analyze the presence of noise [14]. The SNR of biosensor involved in measuring of the limit of detection. ... Conference Paper Full-text available Nano-sphere biosensor is an effective biosensing device used in the applications such as detection of various diseases, safety monitoring of food, diagnosis of cancer. In this work the modeling and analysis of nanosphere biosensor is carried out at a wavelength of 620nm. Finite element method is used for modeling. Light confinement of the biosensor is analyzed along with the analyte concentration and density of transient captured target molecule in analytical method. The SNR of 1.78691 is obtained for 1 × 10 11 cm −2 receptor density, the lower value of signal to noise ratio indicates that the device simulated provides good response to the biomolecules under detection ... Therefore, FETtype sensors are expected to perform better than the resistor-type gas sensor when SNR plays an important role, such as detecting a very low concentration of gas. In addition, the resistor-type gas sensor has a much larger size than the FET type sensor, and the difference in the SNR is expected to be larger considering that the SNR of the sensor is proportional to the square root of the channel area [55]. We believe that the results in this work provide the fundamental information in designing resistor-and FET-type gas sensors with optimal sensing and noise characteristics. ... Article By analyzing the Low Frequency Noise (LFN) characteristics of the resistor-type and the Si Metal Oxide Semiconductor Field Effect Transistor (FET)-type gas sensors fabricated on the same wafer, the intrinsic device noise and the additional noise generated from the gas reaction are systemically examined. Sensing material, n-type Indium-Oxide (In2O3) film, is deposited using the radio frequency magnetron sputtering method. Unlike the FET-type gas sensor, the LFN characteristics of the resistor-type gas sensor are affected by the deposition condition of the sensing material. It is shown that the FET-type sensor has at least 10 times less LFN power than the resistor-type gas sensor despite its smaller size. Gas to Air Noise Ratio (GANR) is introduced as a new figure of merit to evaluate and compare the LFN characteristics during the gas reaction in both resistor- and FET-type gas sensors with the sensing layer prepared by different process conditions. The GANRs of the resistor-type sensors range from ∼2 to 4, which demonstrates that the reaction between the gas molecules and the sensing material generates a fluctuation that exceeds the intrinsic noise of devices. However, the FET-type sensors have a constant value of GANR (∼1) regardless of the operation region, showing that the FET-type gas sensors have better performance in terms of noise. ... N T is the trap density including surface states and interface traps/defects. The LOD follows the minimum detectable surface potential change and it is given by 1/SNR, which is limited by the flat-band voltage fluctuations due to the effects of traps and interface states [39,40]. Figure 14b shows the plot of SNR as a function of solution gate voltage. ... Article Full-text available Attributable to the rapid increase in human infection of Coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), the World Health Organization (WHO) has declared this disease outbreak as a pandemic. This outbreak can be tackled to some extent through proper management and early diagnosis. This work reports a biosensor based on vertical tunnel field-effect transistor (VTFET) developed for the detection of SARS-CoV-2 from the clinical samples through the analysis of its spike, envelope, and DNA proteins. Investigation of the sensitivity of the proposed sensor has been done by calculating the shift in drain current. The dielectric constant equivalent of the virus proteins is used to represent the hybridized biomolecules within the nanogaps. The sensitivity of this proposed sensor is found to be significantly high (order of 10⁶) showing the viability of the device to be a superior sensor. Furthermore, the sensitivity analysis concerning DNA charge density is also performed. The effect of DNA charge density variation on the threshold voltage (Vth) and sensitivity have also been studied in this work. The proposed sensor is also investigated for its noise performance and observed the sensitivity with and without the effect of interface trap charges. Finally, the proposed sensor is benchmarked against the sensitivity of various FET-based biosensors already published earlier. Article In this work, we investigate how the sensitivity of a nanowire or nanoribbon sensor is influenced by the subthreshold slope of the sensing transistor. Polysilicon nanoribbon sensors are fabricated with a wide range of subthreshold slopes and the sensitivity is characterized using pH measurements. It is shown that there is a strong relationship between the sensitivity and the device subthreshold slope. The sensitivity is characterized using the current sensitivity per pH, which is shown to increase from 1.2% to 33.6% as the subthreshold slope improves from 6.2 V/dec to 0.23 V/dec respectively. We propose a model that relates current sensitivity per pH to the subthreshold slope of the sensing transistor. The model shows that sensitivity is determined only on the sub-threshold slope of the sensing transistor and the choice of gate insulator. The model fully explains the values of current sensitivity per pH for the broad range of subthreshold slopes obtained in our fabricated nanoribbon devices. It is also able to explain values of sensitivity reported in the literature, which range from 2.5%/pH to 650%/pH for a variety of nanoribbon and nanowire sensors. For the first time, the proposed model therefore provides a figure-of-merit for comparing the performance of nanoscale field effect transistor sensors fabricated using different materials and technologies. Article This letter presents a microelectrode cell dedicated to direct assessment of the solid-liquid interface noise without recourse to a reference electrode. In the present design, two identical TiN electrodes of various sizes are used for differential measurements in KCl-based electrolytes. Measured noise of the TiN|electrolyte system is found to be of thermal nature. Scaling inversely with electrode area, the noise is concluded to mainly arise from the solid-liquid interface. This noise is comparable to or larger than that of the state-of-the-art MOSFETs. Therefore, its influence cannot be overlooked for the design of future ion sensors. Article Interfacing nanoelectronic devices with cell membranes can enable multiplexed detection of fundamental biological processes (such as signal transduction, electrophysiology, and import/export control) even down to the single ion channel level, which can lead to a variety of applications in pharmacology and clinical diagnosis. Therefore, it is necessary to understand and control the chemical and electrical interface between the device and the lipid bilayer membrane. Here, we develop a simple bottom-up approach to assemble tethered bilayer lipid membranes (tBLM) on silicon wafers and glass slides, using a covalent tether attachment chemistry based on silane functionalization, followed by step-by-step stacking of two other functional molecular building blocks (oligo-PEG and lipid). A standard vesicle fusion process was used to complete the bilayer formation. The monolayer synthetic scheme includes three well-established chemical reactions: self-assembly, epoxy-amine reaction, and EDC/NHS cross-linking reaction. All three reactions are facile and simple, which could be easily implemented in many research labs based on common, commercially available precursors using mild reaction conditions. The oligo-PEG acts as the hydrophilic spacer, a key role in the formation of a homogeneous bilayer membrane. To explore the broad applicability of this approach, we have further demonstrated the formation of tBLMs on three common classes of (nano-) electronic biosensor devices: indium-tin oxide (ITO) coated glass, silicon nanoribbon devices, and high-density single walled nanotube (SWNT) networks on glass. More importantly, we incorporated alemethicin into tBLMs and realized the real-time recording of single ion channel activity with high sensitivity and high temporal resolution using the tBLMs/SWNT network transistor hybrid platform. This approach can provide a covalently-bonded lipid coating on the oxide layer of nanoelectronic devices, which will enable a variety of applications in the emerging field of nanoelectronic interfaces to electrophysiology. Article In this work, we investigate the effects of IGZO film thickness on the H2S gas sensing performance, such as response, excess recovery, low-frequency noise (LFN), and signal-to-noise ratio (SNR) in the resistor- and FET-type gas sensors. A transition of the dominant sensing area from the surface to the bulk of the IGZO film is observed with increasing film thickness. Also, excessive recovery is observed and its detailed mechanism is analyzed for the first time. The resistor-type gas sensor with thicker IGZO film has a smaller 1/f noise due to the decreased trap density and impurity scattering, which guarantees the largest SNR and the lowest limit of detection (LOD). In the case of the FET-type gas sensor, the thicker film with high porosity allows H2S gas to diffuse more easily into the IGZO-O/N/O interface and increases the response. Also, the LFN characteristics of the FET-type gas sensor have no dependence on the IGZO film thickness. Thus, the SNR of the FET-type gas sensor is the largest when the IGZO film thickness is 60 nm. Article Sensing performances of the Si-based field-effect transistor (FET)-type gas sensor are not only affected by sensing material characteristics but also by electrical properties of a transducer. Therefore, the optimization of transducer properties, including subthreshold swing, transconductance, and low-frequency noise (LFN) characteristics, is necessary for improving the sensing performances. In this paper, NO2 gas sensing properties, LFN characteristics, and signal-to-noise ratio (SNR) of the FET-type gas sensor having different channel structures (surface and buried channel FETs) are investigated. An n-type indium-gallium-zinc oxide (IGZO) thin film is used as a sensing layer. The LFN characteristics of both sensors are explained using a carrier number fluctuation model with correlated mobility fluctuation. The flat-band voltage fluctuation (SVfb) value of the buried channel (4.54×10⁻¹⁰ V²/Hz) is smaller than that of the surface channel (2.73×10⁻⁹ V²/Hz). Thus, the SNR of the sensor with a buried channel shows ~10 times larger SNR than that with a surface channel. Also, the optimal bias conditions for both sensors are suggested. The sensor with buried channel FET has a limit of detection (LOD) of 44.2 ppt to NO2 gas. Article Organic electrochemical transistors (OECTs) are increasingly studied as transducers in sensing applications. While much emphasis has been placed on analyzing and maximizing the OECT signal, noise has been mostly ignored, although it determines the resolution of the sensor. The major contribution to the noise in sensing devices is the 1/f noise, dominant at low frequency. In this work, we demonstrate that the 1/f noise in OECTs follows a charge-noise model, which reveals that the noise is due to charge fluctuations in proximity or within the bulk of the channel material. We present the noise scaling behavior with gate voltage, channel dimensions, and polymer thickness. Our results suggest the use of large area channels in order to maximize the signal-to-noise ratio (SNR) for biochemical and electrostatic sensing applications. A comparison with the literature shows that the magnitude of the noise in OECTs is similar to that observed in graphene transistors, and only slightly higher than that found in carbon nanotubes and silicon nanowire devices. In a model ion-sensing experiment with OECTs, we estimate crucial parameters such as the characteristic SNR and the corresponding limit of detection. Article We analyzed the low-frequency noise (LFN) of dual-gated field-effect transistor (DG-FET) biosensors with Schottky contacts. We found the flicker noise at the sensing insulator–semiconductor interface to be the major noise source while employing Schottky contacts to have minimal noise contribution with a sufficiently large back-gate bias voltage. The measured noise dependence on transconductance further indicated the presence of a nonuniform energy distribution of interface trap density at the said sensing interface. Based on these findings, we argued that the DG structure is advantageous over its single-gated (SG) counterpart; although they possess the same intrinsic lower limit of detection (LLOD), the former could offer a larger signal gain at the optimum LLOD thanks to sufficient channel carrier supply through back-gating instead of biasing the sensing interface toward band edge with higher trap density. Article In this work, the low-frequency noise (LFN) characteristics of reconfigurable field-effect-transistor (RFET), gated Schottky Diode (GSD), are investigated. The GSD has a different 1/f noise origin depending on the operation mode (p- or n-type FET mode) and current (IO) region (low or high IO region). Depending on the 1/f noise origin, the effects of the program/erase (P/E) cycling differs. The n-type mode GSD operating in the high IO region whose 1/f noise is originated from the carrier number fluctuation (CNF) shows the largest noise increase after the 104 P/E cycling. Article Ultra-sensitive field-effect transistor-based biosensors using quasi-two dimensional metal oxide semiconductors were demonstrated. Quasi-two dimensional low-dimensional metal oxide semiconductor was highly sensitive to electrical perturbations at the semiconductor-bio interface and showed the competitive sensitivity compared with other nano materials-based biosensors. Also, solution process endowed our platform simple process and high reproducibility, which was favorable compared with other nanobioelectronics. A quasi-two dimensional In2O3-based pH sensor showed a small detection limit of 0.0005 pH and detected the glucose concentration of femtomolar levels. Detailed electrical characterization unveiled how the device’s parameters affect the biosensor sensitivity and lowest detectable charge was extrapolated, which was consistent with the experimental data. Article Planar platinum silicide (PtSi) Schottky barrier (SB) p-channel silicon-on-insulator (SOI) ion-sensitive field effect transistors (ISFETs) have been fabricated at low temperature (400 °C) for biosensor applications. The use of PtSi not only eliminates the ion implantation process and associated high temperature annealing, it also simplifies the process complexity. Both DC and low frequency noise have been characterized. It is found that the SB p-channel SOI ISFET shows a high sensitivity of 55 mV/pH and a resolution limit of 0.0136% of a pH shift with 1 Hz bandwidth. All these suggest that the low temperature PtSi SB SOI ISFET is one of the promising candidates for biochemical detection. Article Layered rhenium disulfide (ReS2) field effect transistors (FETs), with thickness ranging from few to dozens of layers, are demonstrated on 20 nm-thick HfO2/Si substrates. A small threshold voltage of -0.25 V, high on/off current ratio of up to ~10⁷, small subthreshold swing of 116 mV/dec, and electron carrier mobility of 6.02 cm²/V·s are obtained for the two-layer ReS2 FETs. Low frequency noise characteristics in ReS2 FETs are analyzed for the first time and it is found that the carrier number fluctuation mechanism well describes the flicker (1/f) noise of ReS2 FETs with different thicknesses. pH sensing using two-layer ReS2 FET with HfO2 as sensing oxide is then demonstrated with a voltage sensitivity of 54.8 mV/pH and a current sensitivity of 126. The noise characteristics of the ReS2 FET based pH sensors are also examined and a corresponding detection limit of 0.0132 pH is obtained. Our studies suggest the high potential of ReS2 for future low-power nanoelectronics and biosensor applications. Article This paper presents a noise spectroscopy analysis of the current traces recorded in a functionalized silicon oxide nanopore in presence of specific antigen (Hep-B), nonspecific antigen (BSA), and their complex mixture for the first time. It is observed that though the on and off dwell times can differentiate nonspecific antigen from specific antigen in pure buffer, an approximate quantification of the specific antigen with low-dissociation constant of the receptor–ligand pair, becomes almost impossible in complex mixture. This has been ascribed to the significant overlap in the current blockade sensitivity values between the different concentration ranges of the specific antigen. On the contrary, a noise spectroscopy analysis shows a Lorentzian spectrum in presence of specific antigen with a distinct shift in the roll-off frequency, such that upto 1-nM BSA concentration; it has been possible to estimate the concentration of the specific antigen even for 1-pM Hep-B. However, for BSA concentration greater than 1 nM, the roll-off frequency for a particular concentration of specific antigen starts deviating from its value in pure buffer and overlaps with other concentration range. This problem has been addressed by processing the fractional change in current blockade and roll-off frequency by a partial least square-discriminant analysis based multivariate statistical model. It has been observed that the learning model yields 91.5% correct classification with the solutions, and has been able to predict the concentration of Hep-B quite closely even for a low value of 1 pM in presence of 100 nM concentration of BSA. Article Worldwide infection and fatality by SARS-CoV-19 virus and its variants responsible for COVID 19 have impeded economic growth of developing nations beyond repair, General public in several nations have lost their livelihood, it has left severely impacted international relations and most importantly health infrastructures across the world have been tormented. This pandemic has already left footprints on human psychology, traits, and priorities and is certainly going to lead towards new world order in time to come. As always, science and technology come to rescue the human race. The prevention of infection by instant and repeated cleaning of surfaces which are most likely to be touched in daily life and sanitization drives using medically prescribed sanitizers and UV exposure of textiles are the first steps to break the chain. However, the real challenge is to develop and uplift medical infrastructure such as diagnostic tools capable of prompt diagnosis, instant and economic medical treatment available to masses. Two dimensional (2D) materials such as graphene are atomic sheets which have been in news from quite some time due to unprecedented electronic mobilities, high thermal conductivity, appreciable thermal stability, excellent anchoring capabilities, their optical transparency, mechanical flexibility and their unique capability to integrate to arbitrary surfaces. These attributes of 2D materials make them lucrative for their use as active materials platform for authentic and prompt (within minutes) disease diagnosis via electrical or optical diagnostic tools or via electrochemical diagnosis. We present the opportunities provided by 2D materials as materials platform for COVID 19 diagnosis. Article In the field of gas sensor studies, most researchers are focusing on improving the response of the sensors to detect a low concentration of gas. However, factors that make a large response, such as abundant or strong adsorption sites, also work as a source of noise, resulting in a trade-off between response and noise. Thus, the response alone cannot fully evaluate the performance of sensors, and the signal-to-noise-ratio (SNR) should additionally be considered to design gas sensors with optimal performance. In this regard, thin-film-type sensing materials are good candidates thanks to their moderate response and noise level. In this paper, we investigate the effects of radio frequency (RF) sputtering power for deposition of sensing materials on the SNR of resistor- and field-effect transistor (FET)-type gas sensors fabricated on the same Si wafer. In the case of resistor-type gas sensors, the deposition conditions that improve the response also worsen the noise either by increasing the scattering at the bulk or damaging the interface of the sensing material. Among resistor-type gas sensors with sensing materials deposited with different RF powers, a sensor with low noise shows the largest SNR despite its small response. However, the noise of FET-type gas sensors is not affected by changes in RF power and thus there is no trade-off between response and noise. The results reveal different noise sources depending on the deposition conditions of the sensing material, and provide design guidelines for resistor- and FET-type gas sensors considering noise for optimal performance. Article This paper presents the strategy of selective-area growing of a positively-charged layer of Al2O3 and the negatively-charged layer of HfO2 on ion beam track-etched polyethylene terephthalate (PET) nanotubes through the thermal atomic layer deposition (T-ALD) technique. We used the self-assembled monolayer of octadecyl trichlorosilane (OTS-SAMs) on the surface to serve as a passivation layer and then selectively deposited the Al2O3 and HfO2 in the nanotubes. The influence of the dipping time of the substrates in the OTS solution and the experimental conditions on the roughness and the thickness of the OTS monolayer have been investigated. X-ray photoelectron spectroscopy (XPS) was used to analyze the composition of the ALD Al2O3 and HfO2 films. Atomic force microscope (AFM) and scanning electron microscope (SEM) were employed to study the morphologies before and after the ALD of Al2O3 and HfO2. The I-V characteristics of the film confirmed the surface charge polarities in the nanotubes, i.e. the positively-charged Al2O3 and negatively-charged HfO2, in the electrically-neutral solution. The results will aid surface modification and functionalization of PET by nanotubes. Article We have demonstrated atomically thin, quantum capacitance-limited, field-effect transistors (FETs) that enable the detection of pH changes with 75-fold higher sensitivity (~4.4 V/pH) over the Nernst value of 59 mV/pH at room temperature when used as a biosensor. The transistors, which are fabricated from monolayer films of MoS2, use a room temperature ionic liquid (RTIL) in place of a conventional oxide gate dielectric and exhibit very low intrinsic noise resulting in a pH resolution of 92x10^-6 at 10 Hz. This high device performance, which is a function of the structure of our device, is achieved by remotely connecting the gate to a pH sensing element allowing the FETs to be reused. Because pH measurements are fundamentally important in biotechnology, the increased resolution demonstrated here will benefit numerous applications ranging from pharmaceutical manufacturing to clinical diagnostics. As an example, we experimentally quantified the function of the kinase Cdk5, an enzyme implicated in Alzheimer’s disease, at concentrations that are 5-fold lower than physiological values, and with sufficient time-resolution to allow the estimation of both steady-state and kinetic parameters in a single experiment. The high sensitivity, increased resolution, and fast turnaround time of the measurements will allow the development of early diagnostic tools and novel therapeutics to detect and treat neurological conditions years before currently possible. Article The design of the FET-type gas sensor affects various sensor performance factors such as sensitivity, noise, and power consumption. However, few studies comprehensively consider the impact of sensor design on the factors. Here, we show how the design of FET-type gas sensors influences a variety of performance factors and provide design guidelines for FET-type gas sensors. Sensors with several sensing material areas and FET channel areas are fabricated, and fabricated sensors are modeled using Technology Computer-Aided Design (TCAD) simulation to obtain the coupling ratio between the sensing material and FET channel and to investigate the gas response mechanism of the FET-type sensor. As the area of the sensing material increases, the sensitivity of the sensor increases due to the increase in the coupling ratio. Meanwhile, the area to be heated by the micro-heater also increases, thereby increasing power consumption. As the FET channel area decreases, the coupling ratio increases, resulting in better sensitivity, but the noise of the sensor increases. Therefore, it is desirable to design the FET-type gas sensor using sensitivity/power and signal-to-noise ratio (SNR) as indicators in consideration of sensitivity, power consumption, and noise. A FET-type gas sensor can be optimized by considering the performance factors of trade-offs, setting an optimization indicator suitable for the application, and then selecting design parameters that maximize the indicator. As a proof of concept, we show processes to optimize the horizontal floating-gate FET (HFGFET)-type gas sensor using ΔID/power, SNR, SNR/power, and (SNR/power)/size as optimization indicators. Article Full-text available Transistor biosensors are mass-fabrication-compatible devices of interest for point of care diagnosis as well as molecular interaction studies. While the actual transistor gates in processors reach the sub-10 nm range for optimum integration and power consumption, studies on design rules for the signal-to-noise ratio (S/N) optimization in transistor-based biosensors have been so far restricted to 1 µm2 device gate area, a range where the discrete nature of the defects can be neglected. In this study, which combines experiments and theoretical analysis at both numerical and analytical levels, we extend such investigation to the nanometer range and highlight the effect of doping type as well as the noise suppression opportunities offered at this scale. In particular, we show that, when a single trap is active near the conductive channel, the noise can be suppressed even beyond the thermal limit by monitoring the trap occupancy probability in an approach analog to the stochastic resonance effect used in biological systems. Article W-doped NiO nanotubes (NTs) were successfully synthesized for efficient sensing of ethanol vapor using a low cost and economical electrospinning method. The response (resistance ratio) of sensor based on 4 % W-NiO NTs to100 ppm ethanol is as high as 40.56 at 200 °C, which is 10 times higher than that of pure NiO sensor. This significant enhancement in sensor response can be attributed to the change in morphology, increase in oxygen vacancies, and the decrease in hole concentration in NiO by W doping. Based on the adsorption and desorption theory of oxygen, we also demonstrated that the atmospheric oxygen content can improve the sensor response and shorten the response time by increasing the amount of adsorbed oxygen. Article Full-text available Since the isolation of graphene in 2004, there has been an exponentially growing number of reports on layered two-dimensional (2D) materials for applications ranging from protective coatings to biochemical sensing. Due to the exceptional, and often tunable, electrical, optical, electrochemical, and physical properties of these materials, they can serve as the active sensing element or a supporting substrate for diverse healthcare applications. In this review, we provide a survey of the recent reports on the applications of 2D materials in biosensing and other emerging healthcare areas, ranging from wearable technologies to optogenetics to neural interfacing. Specifically, this review provides (i) a holistic evaluation of relevant material properties across a wide range of 2D systems, (ii) comparison of 2D material-based biosensors to the state-of-the-art, (iii) relevant material synthesis and functionalization approaches specifically reported for healthcare applications, and (iv) the technological considerations to facilitate mass production and commercialization. Article Full-text available We demonstrate an approach for highly sensitive bio-detection based on silicon nanowire field-effect transistors by employing low frequency noise spectroscopy analysis. The inverse of noise amplitude of the device exhibits an enhanced gate coupling effect in strong inversion regime when measured in buffer solution than that in air. The approach was further validated by the detection of cardiac troponin I of 0.23 ng/ml in fetal bovine serum, in which 2 orders of change in noise amplitude was characterized. The selectivity of the proposed approach was also assessed by the addition of 10 μg/ml bovine serum albumin solution. Article Full-text available High-performance dual-gate (DG) ion-sensitive field-effect transistors (ISFETs) beyond the Nernstian limit of 59 mV/pH were realized using the fully depleted (FD) silicon-on-insulator (SOI) substrate. The FD SOI-based DG ISFET exhibited a significantly enhanced pH sensitivity of 379.2 mV/pH for DG operation amplified by capacitive coupling, while it exhibited a relatively poor sensitivity of 47.9 mV/pH for single-gate (SG) operation. Meanwhile, the non-ideal effects for long-term use slightly increased by the DG operation compared to the SG operation. Therefore, the FD SOI-based DG ISFETs compatible with the complementary metal-oxide-semiconductor process are considered to be very promising bio-chemical sensors. Article Full-text available Nanowire transistors are promising candidates for future electronics applications; however, they generally exhibit higher levels of low-frequency noise compared with traditional MOSFETs. The physics of this noise generation in nanowires needs to be understood for improving the device performance. In this paper, the low-frequency noise in InAs nanowire transistors was studied at different gate voltages before and after passivation by a polymethyl methacrylate (PMMA) layer. Noise levels in nanowire devices can be separated into contributions from the channel and from the contacts by analyzing the noise behavior under different bias conditions for devices with varying channel lengths. It is shown that a noise component, which is independent of channel length, can be attributed to the contacts, and a length-dependent component is attributed to the channel. Applying the PMMA passivation layer over the entire device reduces the noise level generated by the channel, but does not change the noise level generated by the contacts. This paper provides a method to understand, and potentially improve, the noise performance. Operation in a channel-dominated bias regime allows extraction of a Hooge parameter specifically for the channel. PMMA passivation was effective in reducing this channel Hooge parameter from 1.4 x 10⁻¹ to 1.8 x 10³. Article Full-text available We fabricated ZnO nanowire field effect transistors FETs and systematically characterized their low frequency f noise properties. The obtained noise power spectra showed a classical 1 / f dependence. A Hooge's constant of 5 10 −3 was estimated from the gate dependence of the noise amplitude. This value is within the range reported for complementary metal-oxide semiconductor CMOS FETs with high-k dielectrics, supporting the concept that nanowires can be utilized for future beyond-CMOS electronic applications from the point of view of device noise properties. ZnO FETs measured in a dry O 2 environment displayed elevated noise levels that can be attributed to increased fluctuations associated with O 2 − on the nanowire surfaces. © 2007 American Institute of Physics. Article Full-text available Monitoring the binding affinities and kinetics of protein interactions is important in clinical diagnostics and drug development because such information is used to identify new therapeutic candidates. Surface plasmon resonance is at present the standard method used for such analysis, but this is limited by low sensitivity and low-throughput analysis. Here, we show that silicon nanowire field-effect transistors can be used as biosensors to measure protein-ligand binding affinities and kinetics with sensitivities down to femtomolar concentrations. Based on this sensing mechanism, we develop an analytical model to calibrate the sensor response and quantify the molecular binding affinities of two representative protein-ligand binding pairs. The rate constant of the association and dissociation of the protein-ligand pair is determined by monitoring the reaction kinetics, demonstrating that silicon nanowire field-effect transistors can be readily used as high-throughput biosensors to quantify protein interactions. Article Full-text available The low-frequency (1/ f ) noise of gate-all-around silicon nanowire transistors (SNWTs) with different gate electrodes (poly-Si gate, doped fully silicided (FUSI) gate, and undoped FUSI gate) is studied in the strong-inversion linear region. It shows that the gate electrodes have a strong impact on the 1/ f noise of the SNWTs. The highest noise is observed in the SNWTs with a poly-Si gate, compared to their FUSI-gate counterparts. The observations are explained according to the number fluctuation with correlated mobility fluctuation theory by assuming that the correlated mobility scattering is better screened in the case of an undoped FUSI gate. However, the doped FUSI gate with silicidation-induced impurity segregation at the gate/SiO<sub>2</sub> interface gives rise to extra mobility scattering. Article Full-text available The low frequency (1/f) noise in single Sn O <sub>2</sub> nanowire transistors was investigated to access semiconductor-dielectric interface quality. The amplitude of the current noise spectrum (S<sub>I</sub>) is found to be proportional to I<sub> d </sub><sup>2</sup> in the transistor operating regime. The extracted Hooge’s constants (α<sub>H</sub>) are 4.5×10<sup>-2</sup> at V<sub> ds </sub>=0.1 V and 5.1×10<sup>-2</sup> at V<sub> ds </sub>=1 V , which are in general agreement with our prior studies of nanowire/nanotube transistors characterized in ambient conditions. Furthermore, the effects of interface states and contacts on the noise are also discussed. Article Full-text available The conductance change of nanowire field-effect transistors is considered a highly sensitive probe for surface charge. However, Debye screening of relevant physiological liquid environments challenge device performance due to competing screening from the ionic liquid and nanowire charge carriers. The authors discuss this effect within Thomas-Fermi and Debye-Hückel theory and derive analytical results for cylindrical wires which can be used to estimate the sensitivity of nanowire surface-charge sensors. They study the interplay between the nanowire radius, the Thomas-Fermi and Debye screening lengths, and the length of the functionalization molecules. The analytical results are compared to finite-element calculations on a realistic geometry. Article Full-text available Nanoelectronic devices based upon self-assembled semiconductor nanowires are excellent research tools for investigating the behavior of structures with sublithographic features as well as a promising basis for future information processing technologies. New test structures and associated electrical measurement methods are the primary metrology needs necessary to enable the development, assessment, and adoption of emerging nanowire electronics. We describe two unique approaches to successfully fabricate nanowire devices: one based upon harvesting and positioning nanowires and one based upon the direct growth of nanowires in predefined locations. Test structures are fabricated and electronically characterized to probe the fundamental properties of chemical-vapor-deposition-grown silicon nanowires. Important information about current transport and fluctuations in materials and devices can be derived from noise measurements, and low-frequency$ hbox{1}/f$noise has traditionally been utilized as a quality and reliability indicator for semiconductor devices. Both low-frequency$hbox{1}/f\$ noise and random telegraph signals are shown here to be powerful methods for probing trapping defects in nanoelectronic devices.
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The seminal importance of DNA sequencing to the life sciences, biotechnology and medicine has driven the search for more scalable and lower-cost solutions. Here we describe a DNA sequencing technology in which scalable, low-cost semiconductor manufacturing techniques are used to make an integrated circuit able to directly perform non-optical DNA sequencing of genomes. Sequence data are obtained by directly sensing the ions produced by template-directed DNA polymerase synthesis using all-natural nucleotides on this massively parallel semiconductor-sensing device or ion chip. The ion chip contains ion-sensitive, field-effect transistor-based sensors in perfect register with 1.2 million wells, which provide confinement and allow parallel, simultaneous detection of independent sequencing reactions. Use of the most widely used technology for constructing integrated circuits, the complementary metal-oxide semiconductor (CMOS) process, allows for low-cost, large-scale production and scaling of the device to higher densities and larger array sizes. We show the performance of the system by sequencing three bacterial genomes, its robustness and scalability by producing ion chips with up to 10 times as many sensors and sequencing a human genome.
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Recent studies on nanoscale field-effect sensors reveal the crucial importance of the low frequency noise for determining the ultimate detection limit. In this letter, the 1/f-type noise of Si nanoribbon field-effect sensors is investigated. We demonstrate that the signal-to-noise ratio can be increased by almost two orders of magnitude if the nanoribbon is operated in an optimal gate voltage range. In this case, the additional noise contribution from the contact regions is minimized, and an accuracy of 0.5% of a pH shift in one Hz bandwidth can be reached.
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Boron-doped silicon nanowires (SiNWs) were used to create highly sensitive, real-time electrically based sensors for biological and chemical species. Amine- and oxide-functionalized SiNWs exhibit pH-dependent conductance that was linear over a large dynamic range and could be understood in terms of the change in surface charge during protonation and deprotonation. Biotin-modified SiNWs were used to detect streptavidin down to at least a picomolar concentration range. In addition, antigen-functionalized SiNWs show reversible antibody binding and concentration-dependent detection in real time. Lastly, detection of the reversible binding of the metabolic indicator Ca2+ was demonstrated. The small size and capability of these semiconductor nanowires for sensitive, label-free, real-time detection of a wide range of chemical and biological species could be exploited in array-based screening and in vivo diagnostics.
The advantage of SOI wafers for device manufacture has been widely studied. To be a real challenger to bulk silicon, SOI producers have to offer SOI wafers in large volume and at low cost. The new Smart‐Cut® SOI process used for the manufacture of the Unibond® SOI wafers answers most of the SOI wafer manufacturability issues. The use of Hydrogen implantation and wafer bonding technology is the best combination to get good uniformity and high quality for both the SOI and buried oxide layer. In this paper, the Smart‐Cut® process is described in detail and material characteristics of Unibond® wafers such as crystalline quality, surface roughness, thin film thickness homogeneity, and electric behavior.
Article
The theory of surface state effects on noise in p−n junctions is extended to explain low-frequency excess noise in MOS transistors. Experimental results are critically compared to theory. The results show that low-frequency excess noise in MOS transistors is due to the fluctuation of charge density in the conduction channel caused by the surface potential fluctuation. The fluctuation of the surface potential is introduced by the random charge occupancy of surface states. The low-frequency excess noise in MOS transistors is found to be proportional to the surface state density and the square of the transconductance of the device, and inversely proportional to the gate area and the square of the unit area gate-insulator capacitance. It is also shown that the surface state density when the surface is strongly inverted can be obtained from noise measurements. Finally it is shown that by proper heat treatment it is possible to reduce the low-frequency excess noise of MOS transistors.
Article
The interactions between metal oxide nanowires and molecular species can significantly affect the electrical properties of metal oxide nanowires. A passivation process is needed to stabilize the electrical characteristics, regardless of the environmental changes. Herein, we investigated the passivation effects of a polymethyl methacrylate (PMMA) layer on SnO2 nanowire (NW) field-effect transistors (FETs). As a result of the PMMA coating, the electrical properties of the SnO2 NW FETs improved. The electrical noise behavior in both non-passivated and passivated devices can be described with the carrier number fluctuation model associated with the trapping and the release of charge carriers at the surface. The non-passivated devices exhibited higher noise levels than those of the passivated devices. These results demonstrate that surface passivation can lead to the suppression of dynamic responses (electron trapping/release events and scattering fluctuations).
Article
Field-effect sensors used to detect and identify biological species have been proposed as alternatives to other methods such as fluorescence deoxyribonucleic acid (DNA) microarrays. Sensors fabricated using commercial complementary metal-oxide-semiconductor technology would enable low-cost and highly integrated biological detection systems. In this paper, the small-signal and noise modeling of biosensors implemented with electrolyte-insulator-semiconductor structures is studied, with emphasis on design guidelines for low-noise performance. In doing so, a modified form of the general charge sheet metal-oxide-semiconductor field-effect transistor model that better fits the electrolyte-insulator-semiconductor structure is used. It is discussed how if the reference electrode and the insulator-electrolyte generate no noise associated with charge transport, then the main noise mechanisms are the resistive losses of the electrolyte and the low-frequency noise of the field-effect transistor. It is also found that for realistic sensor geometries and high electrolyte concentrations, the noise from the field-effect transistor (FET) dominates the thermal noise from the electrolyte resistance, and the optimal biasing point for the FET for minimum noise is found to be around moderate inversion.
Article
The maximum sensitivity of classical nanowire (NW)-based pH sensors is defined by the Nernst limit of 59 mV/pH. For typical noise levels in ultra-small single-gated nanowire sensors, the signal-to-noise ratio is often not sufficient to resolve pH changes necessary for a broad range of applications. Recently, a new class of double-gated devices was demonstrated to offer apparent "super-Nernstian" response (>59 mV/pH) by amplifying the original pH signal through innovative biasing schemes. However, the pH-sensitivity of these nanoscale devices as a function of biasing configurations, number of electrodes, and signal-to-noise ratio (SNR) remains poorly understood. Even the basic question such as "Do double-gated sensors actually resolve smaller changes in pH compared to conventional single-gated sensors in the presence of various sources of noise?" remains unanswered. In this article, we provide a comprehensive numerical and analytical theory of signal and noise of double-gated pH sensors to conclude that, while the theoretical lower limit of pH-resolution does not improve for double-gated sensors, this new class of sensors does improve the (instrument-limited) pH resolution.
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Semiconductor nanowires have achieved great attention for integration in next-generation electronics. However, for nanowires with diameters comparable to the Debye length, which would generally be required for one-dimensional operation, surface states degrade the device performance and increase the low-frequency noise. In this study, single In(2)O(3) nanowire transistors were fabricated and characterized before and after surface passivation with a self-assembled monolayer of 1-octadecanethiol (ODT). Electrical characterization of the transistors shows that device performance can be enhanced upon ODT passivation, exhibiting steep subthreshold slope (~64 mV/dec), near zero threshold voltage (~0.6 V), high mobility (~624 cm(2)/V·s), and high on-currents (~40 μA). X-ray photoelectron spectroscopy studies of the ODT-passivated nanowires indicate that the molecules are bound to In(2)O(3) nanowires through the thiol linkages. Device simulations using a rectangular geometry to represent the nanowire indicate that the improvement in subthreshold slope and positive shift in threshold voltage can be explained in terms of reduced interface trap density and changes in fixed charge density. Flicker (low-frequency, 1/f) noise measurements show that the noise amplitude is reduced following passivation. The interface trap density before and after ODT passivation is profiled throughout the band gap energy using the subthreshold current-voltage characteristics and is compared to the values extracted from the low-frequency noise measurements. The results indicate that self-assembled monolayer passivation is a promising optimization technology for the realization of low-power, low-noise, and fast-switching applications such as logic, memory, and display circuitry.
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The 1/f noise of silicon nanowire (NW) biological field-effect transistors (NW FETs with exposed channels) is characterized and compared with various fabrication approaches, specifically, a wet orientation-dependent etch (ODE) versus common plasma-based etching methods. The wet-etched devices are shown to have significantly lower noise and subthreshold swing, and the average extracted Hooge parameter for ODE wet-etched devices (α<sub>H</sub> = 2.1 × 10<sup>-3</sup>) is comparable to the values reported for submicrometer MOSFETs with a metal/HfO<sub>2</sub> gate stack.
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A brief overview of recent issues concerning the low frequency (LF) noise in modern CMOS devices is given. The approaches such as the carrier number and the Hooge mobility fluctuations used for the analysis of the noise sources are presented and illustrated through experimental results obtained on advanced CMOS generations. The use of the LF noise measurements as a characterization tool of large area MOS devices is also discussed. The main physical features of random telegraph signals (RTSs) observed in small area MOS transistors are reviewed. The impact of scaling on the LF noise and RTS fluctuations in CMOS silicon devices is also addressed. Experimental results obtained on 0.18 μm CMOS technologies are used to predicting the trends for the noise figure of foregoing CMOS technologies e.g. 0.1 μm and beyond. The formulation of the thermal noise underlying the LF fluctuations in MOSFETs is recalled for completeness.
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The signal-to-noise ratio (SNR) for silicon nanowire field-effect transistors operated in an electrolyte environment is an essential figure-of-merit to characterize and compare the detection limit of such devices when used in an exposed channel configuration as biochemical sensors. We employ low frequency noise measurements to determine the regime for optimal SNR. We find that SNR is not significantly affected by the electrolyte concentration, composition, or pH, leading us to conclude that the major contributions to the SNR come from the intrinsic device quality. The results presented here show that SNR is maximized at the peak transconductance.
Article
Nanoscale ISFET (ion sensitive field-effect transistor) pH sensors are presented that produce the well-known sub-nernstian pH-response for silicon dioxide (SiO(2)) surfaces and near ideal nernstian sensitivity for alumina (Al(2)O(3)) surfaces. Titration experiments of SiO(2) surfaces resulted in a varying pH sensitivity ∼20 mV/pH for pH near 2 and >45 mV/pH for pH > 5. Measured pH responses from titrations of thin (15 nm) atomic layer deposited (ALD) alumina (Al(2)O(3)) surfaces on the nanoISFETs resulted in near ideal nernstian pH sensitivity of 57.8 ± 1.2 mV/pH (pH range: 2-10; T = 22 °C) and temperature sensitivity of 0.19 mV/pH °C (22 °C ≤ T ≤ 40 °C). A comprehensive analytical model of the nanoISFET sensor, which is based on the combined Gouy-Chapman-Stern and Site-Binding (GCS-SB) model, accompanies the experimental results and an extracted ΔpK ≈ 1.5 from the measured responses further supports the near ideal nernstian pH sensitivity.
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Field effect transistors (FETs) are widely used for the label-free detection of analytes in chemical and biological experiments. Here we demonstrate that the apparent sensitivity of a dual-gated silicon nanowire FET to pH can go beyond the Nernst limit of 60 mV/pH at room temperature. This result can be explained by a simple capacitance model including all gates. The consistent and reproducible results build to a great extent on the hysteresis- and leakage-free operation. The dual-gate approach can be used to enhance small signals that are typical for bio- and chemical sensing at the nanoscale.
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The signal-to-noise ratio (SNR) for real-time biosensing with liquid-gated carbon nanotube transistors is crucial for exploring the limits of their sensitivity, but has not been studied thus far. Although biosensing is often performed at high transconductance where the device displays the largest gate response, here we show that the maximum SNR is actually obtained when the device is operated in the subthreshold regime. In the ON-state, additional contributions to the noise can lead to a reduction of the SNR by up to a factor of 5. For devices with passivated contact regions, the SNR in ON-state is even further reduced than for bare devices. We show that when the conductivity of the contact regions can be increased using a conventional back gate, the SNR in the ON-state can be improved. The results presented here demonstrate that biosensing experiments are best performed in the subthreshold regime for optimal SNR.
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Silicon nanowires of different widths were fabricated in silicon on insulator (SOI) material using conventional process technology combined with electron-beam lithography. The aim was to analyze the size dependence of the sensitivity of such nanowires for biomolecule detection and for other sensor applications. Results from electrical characterization of the nanowires show a threshold voltage increasing with decreasing width. When immersed in an acidic buffer solution, smaller nanowires exhibit large conductance changes while larger wires remain unaffected. This behavior is also reflected in detected threshold shifts between buffer solutions of different pH, and we find that nanowires of width >150 nm are virtually insensitive to the buffer pH. The increased sensitivity for smaller sizes is ascribed to the larger surface/volume ratio for smaller wires exposing the channel to a more effective control by the local environment, similar to a surrounded gate transistor structure. Computer simulations confirm this behavior and show that sensing can be extended even down to the single charge level.
Article
Biosensors based on silicon nanowires (Si-NWs) promise highly sensitive dynamic label-free electrical detection of biomolecules. Despite the tremendous potential and promising experimental results, the fundamental mechanism of electrical sensing of biomolecules and the design considerations of NW sensors remain poorly understood. In this paper, we discuss the prospects and challenges of biomolecule detection using Si-NW biosensors as a function of device parameters, fluidic environment, charge polarity of biomolecules, etc., and refer to experimental results in literature to support the nonintuitive predictions wherever possible. Our results indicate that the design of Si nanobiosensor is nontrivial and as such, only careful optimization supported by numerical simulation would ensure optimal sensor performance.
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Low-frequency 1/f noise in Si n-channel MOSFET's is measured from weak to strong inversion, through the relative spectral density of the drain current fluctuations S_{I}_{D}/I^{2}_{D} . Under specific conditions, a plateau is observed in the variations of S_{I}_{D}/I^{2}_{D} versus the gate voltage in weak inversion followed by a steep decrease in strong inversion. A modified trapping noise theory based on the McWhorter's assumptions and valid in all the working regimes is developed to account for this behavior. Excellent agreement is obtained with the variations of several parameters: gate and drain biases, geometry, oxide and depletion capacitance, temperature, and technologies. The influence of fast interface states is particularly studied and is related to the noise variations and the oxide trap densities.
Article
The impact of device scaling on modern MOS technology is discussed in terms of the random telegraph signals and 1/f noise in MOSFET's. In addition to the more obvious effects of enhanced current fluctuations as the device is scaled down, we will show the influence of nonuniform distribution of threshold voltages along the channel in the context of device scaling. The role of fast interface states on the drain current fluctuations is also discussed. It will be shown that, compared to the oxide traps, fast interface states give rise to higher frequency RTS and 1/f noise, and that they become more important for devices operating in weak inversion
Article
The use of 1/ f noise measurements in n-channel MOSFETs to extract the oxide trap density in space and energy near and above the conduction band edge of silicon is investigated. The conventional carrier number fluctuation model of 1/ f noise that attributes 1/ f noise to the trapping and detrapping of inversion layer carriers by oxide traps is reviewed. It is shown that oxide band bending in devices with a nonuniform oxide trap distribution leads to a gate voltage dependence in the magnitude and exponent γ( V <sub>gs</sub>) of the 1/ f <sup>γ</sup> noise spectrum. An extension of the 1/ f noise number fluctuation model that includes both carrier number fluctuations and correlated mobility fluctuations is then studied. Correlated mobility fluctuations are attributed to the coulombic scattering of inversion layer carriers by the fluctuating trapped charge. It is shown that the correlated fluctuation model predicts a gate voltage dependence in the magnitude and exponent γ of the 1/ f <sup>γ</sup> noise spectrum even for a uniform oxide trap distribution. By analyzing the 1/ f noise magnitude and exponent data in n-channel MOSFETs having various oxide thicknesses, both models are used to extract the oxide trap density over a wide range of space and energy