F. Udrea

University of Cambridge, Cambridge, England, United Kingdom

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Publications (373)271.22 Total impact

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    ABSTRACT: In this paper we present a temperature-modulated graphene oxide (GO) resistive humidity sensor that employs complementary-metal-oxide-semiconductor (CMOS) micro-electro-mechanical-system (MEMS) micro-hotplate technology for the monitoring and control of indoor air quality (IAQ). GO powder is obtained by chemical exfoliation, dispersed in water and deposited via ink-jet printing onto a low power micro-hotplate. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) show the typical layered and wrinkled morphology of the GO. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Fourier transform infra-red (FTIR) spectroscopy indicate that the GO flakes possess a significant number of oxygen containing functional groups (epoxy, carbonyl, hydroxyl) extremely attractive for humidity detection. Electro-thermal characterisation of the micro-hotplates shows a thermal efficiency of 0.11 mW/°C, resulting in a sensor DC power consumption of only 2.75 mW at 50 °C. When operated in an isothermal mode, the sensor response is detrimentally affected by significant drift, hysteretic behaviour, slow response/recovery times and hence poor RH level discrimination. Conversely, a temperature modulation technique coupled with a differential readout methodology results in a significant reduction of the sensor drift, improved linear response with a sensitivity of 0.14 mV/%, resolution below 5%, and a maximum hysteresis of ± 5%; response and recovery times equal to 189 � 49 s and 89 � 5 s, respectively. These performance parameters satisfy current IAQ monitoring requirements. We have thus demonstrated the effectiveness of integrating GO on a micro-hotplate CMOS-compatible platform enabling temperature modulation schemes to be easily applied in order to achieve compact, low power, low cost humidity IAQ monitoring.
    Full-text · Article · Jan 2016 · Nanoscale
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    ABSTRACT: The application of plasmonics to thermal emitters is generally assisted by absorptive losses in the metal because Kirchhoff's law prescribes that only good absorbers make good thermal emitters. Based on a designed plasmonic crystal and exploiting a slow-wave lattice resonance and spontaneous thermal plasmon emission, we engineer a tungsten-based thermal emitter, fabricated in an industrial CMOS process, and demonstrate its markedly improved practical use in a prototype non-dispersive infrared (NDIR) gas-sensing device. We show that the emission intensity of the thermal emitter at the CO2 absorption wavelength is enhanced almost 4-fold compared to a standard non-plasmonic emitter, which enables a proportionate increase in the signal-to-noise ratio of the CO2 gas sensor.
    Full-text · Article · Dec 2015 · Scientific Reports
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    ABSTRACT: This study presents a detailed analysis of the Plasma-enhanced chemical vapour deposition (PECVD) silicon-nitride/semiconductor interface of gallium nitride (GaN) transistors through the study of the transfer characteristics of a large-gate-area metal-insulator-semiconductor field-effect transistor (MISFET). Id-Vg measurements were performed on several MISFETs across the wafer and for all of them the authors observed strong hysteresis between forward and reverse sweeps and a double kink. These features indicate the presence of traps beneath the gate electrode. Neither the hysteresis nor the kinks were seen in the measured high-electron-mobility transistor (HEMT) characteristics suggesting that the passivation/semiconductor interface is electrically responsible for them. The transfer characteristics of the MISFET have been reproduced using a technology computer-aided design (TCAD) deck that includes fixed charges and donor traps at the passivation/semiconductor interface. The impact of these charges on the Id-Vg and their influence on the formation of a surface-inversion layer is here explained through extensive TCAD simulations. This study has also been extended to different temperatures between 35 and 75°C to investigate the change in the transfer characteristics at elevated temperatures. It is shown that the hysteresis observed between forward and reverse sweeps and the transconductance decrease significantly with increasing temperature.
    No preview · Article · Dec 2015 · IET Power Electronics
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    ABSTRACT: In this paper, we present the design and characterization of a low-power low-cost infra-red emitter based on a tungsten micro-hotplate fabricated in a commercial 1-μm SOI-CMOS technology. The device has a 250-μm diameter resistive heater inside a 600-μm diameter thin dielectric membrane. We first present electro-thermal and optical device characterization, long term stability measurements, and then demonstrate its application as a gas sensor for a domestic boiler. The emitter has a dc power consumption of only 70 mW, a total emission of 0.8 mW across the 2.5–15-μm wavelength range, a 50% frequency modulation depth of 70 Hz, and excellent uniformity from device-to-device. We also compare two larger emitters (heater size of 600 and 1800 μm) made in the same technology that have a much higher infra-red emission, but at the detriment of higher power consumption. Finally, we demonstrate that carbon nanotubes can be used to significantly enhance the thermo-optical transduction efficiency of the emitter.
    No preview · Article · Dec 2015 · IEEE Sensors Journal
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    ABSTRACT: We report on the integration of inkjet-printed graphene with a CMOS micro-electro-mechanical-system (MEMS) microhotplate for humidity sensing. The graphene ink is produced via ultrasonic assisted liquid phase exfoliation in isopropyl alcohol (IPA) using polyvinyl pyrrolidone (PVP) polymer as the stabilizer. We formulate inks with different graphene concentrations, which are then deposited through inkjet printing over predefined interdigitated gold electrodes on a CMOS microhotplate. The graphene flakes form a percolating network to render the resultant graphene-PVP thin film conductive, which varies in presence of humidity due to swelling of the hygroscopic PVP host. When the sensors are exposed to relative humidity ranging from 10–80%, we observe significant changes in resistance with increasing sensitivity from the amount of graphene in the inks. Our sensors show excellent repeatability and stability, over a period of several weeks. The location specific deposition of functional graphene ink onto a low cost CMOS platform has the potential for high volume, economic manufacturing and application as a new generation of miniature, low power humidity sensors for the internet of things.
    Full-text · Article · Nov 2015 · Scientific Reports
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    ABSTRACT: Dielectrophoretic alignment is found to be a simple and efficient method to deposit the solution prepared ZnO nanowires onto micro hot plate substrates. Due to the strong surface effects, positive temperature coefficient for resistance was encountered with ZnO nanowires in the high temperature range (>250 °C). The response to ammonia (NH3) was evaluated in isothermal and temperature-pulsed operation mode; the relative higher response observed in the latter case demonstrates that the use of this methodology is a good strategy to improve the performance of metal oxide sensors based on nanomaterials. Here, we evaluate the response to NH3 and qualitatively describe the sensing mechanism in temperature-pulsed mode, highlighting the main differences compared to the standard isothermal methodology.
    No preview · Article · Nov 2015 · Sensors and Actuators B Chemical
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    ABSTRACT: In this letter, a trench-insulated gate bipolar transistor (IGBT) design with local charge compensating layers featured at the cathode of the device is presented and analyzed. The superjunction or reduced surface effect proves to be very effective in overcoming the inherited ON-state versus breakdown tradeoff appearing in conventional devices, such as the soft punch through plus or field stop plus (FS+) IGBTs. This design enhances the ON-state performance of the FS+IGBT by increasing the plasma concentration at the cathode side without affecting either the switching performance or the breakdown rating.
    No preview · Article · Aug 2015 · IEEE Electron Device Letters
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    ABSTRACT: The current paper reports on a sonochemical synthesis method for manufacturing nanostructured (typical grain size of 50 nm) SrTi0.6Fe0.4O2.8 (Sono-STFO40) powder. This powder is characterized using X ray-diffraction (XRD), Mössbauer spectroscopy and Scanning Electron Microscopy (SEM), and results are compared with commercially available SrTi0.4Fe0.6O2.8 (STFO60) powder. In order to manufacture resistive oxygen sensors, both Sono-STFO40 and STFO60 are deposited, by dip-pen nanolithography (DPN) method, on an SOI (Silicon-on-Insulator) micro-hotplate, employing a tungsten heater embedded within a dielectric membrane. Oxygen detection tests are performed in both dry (RH = 0%) and humid (RH = 60%) nitrogen atmosphere, varying oxygen concentrations between 1% and 16% (v/v), at a constant heater temperature of 650 °C. The oxygen sensor, based on the Sono-STFO40 sensing layer, shows good sensitivity, low power consumption (80 mW), and short response time (25 s). These performance are comparable to those exhibited by state-of-the-art O2 sensors based on STFO60, thus proving Sono-STFO40 to be a material suitable for oxygen detection in harsh environments.
    Full-text · Article · Jul 2015 · Sensors
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    ABSTRACT: The Bi-mode gate commutated thyristor (BGCT) is a new type of reverse conducting Gate Commutated Thyristor (GCT). This paper focuses on the maximum controllable current capability of BGCTs and proposes new solutions which can increase it. The impact of proposed solutions in the turn-ON and turn-OFF is also assessed. For this analysis, a 2-D mixed mode model for full-wafer device simulations has been developed and utilized.
    No preview · Article · Jul 2015 · IEEE Transactions on Electron Devices
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    DESCRIPTION: Abstract—In this paper we present a novel Silicon-on-Insulator (SOI) complementary metal oxide semiconductor (CMOS) micro-electro-mechanical-system (MEMS) thermal wall shear stress sensor based on a tungsten hot-wire and a single thermopile. Devices were fabricated using a commercial 1 μm SOI-CMOS process followed by a deep reactive ion etching (DRIE) back-etch step to release a silicon dioxide membrane, which mechanically support and thermally isolates heating and sensing elements. The sensors show an electro-thermal transduction efficiency of 50 µW/°C, and very small zero flow off-set. Calibration for wall shear stress measurement in air in the range of 0 - 0.48 Pa was performed using a suction type, 2-D flow wind tunnel. The sensors were found to be extremely sensitive, up to 4 V/Pa for low wall shear stress values. Furthermore, we demonstrate the superior signal to noise ratio (up to five times higher) of a single thermopile readout configuration compared with a double thermopile readout configuration (embedded for comparison purposes within the same device). Finally, we verify that the output of the sensor is proportional to the cube root of the wall shear stress and we propose an accurate semi-empirical formula for its modelling.
    No preview · Research · Jun 2015
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    ABSTRACT: We report here a new physical phenomenon related to contact etch depth in High Voltage Lateral IGBTs (LIGBTs) and propose a design technique to increase yield of LIGBTs in high volume production. We prove for the first time that the contact geometry and placement have direct effect on Collector injection efficiency in LIGBTs. An improved design for 800V LIGBTs obtained by optimising the layout of contact openings is proposed. The new structure resulted in 15% increase in production yield.
    No preview · Article · Jun 2015
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    ABSTRACT: The key sensor requirements for global-scale environmental monitoring are: low cost; low power; high volume production capabilities; miniature size and ubiquitous, self-powered wireless deployment. Based upon these requirements, a new generation of miniature sensors is emerging that employ nano-materials, such as metal oxides, polymers, carbon, graphene - sometimes structured as nanotubes ornanowires. Platform technologies used for depositing these gas sensing materials are commonly based on ceramic substrate, printable polymer, or silicon wafers together with a MEMS processing step. Amongst these, the most promising platform is the silicon substrate. In particular, a silicon substrate can be processed in commercial foundries with integrated CMOS circuits and is considered to be the most attractive option to enable high volume, low cost ubiquitous smart solutions with multi-sensing solutions. In this paper we give some examples of sensors that have been, or are being developed, for commercial environmental monitoring applications. Using CMOS silicon wafers for environmental sensing application offers added benefits that the sensors can be readily incorporated within portable devices such as smartphones, wearables or even in whitegoods including automotive and other purpose-built electronic systems. We also give examples of such sensors and array of sensors and highlight some of the key benefits of using CMOS sensing solutions for future environmental monitoring.
    Full-text · Conference Paper · Jun 2015
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    ABSTRACT: In this paper we present a novel Silicon-on-Insulator (SOI) complementary metal oxide semiconductor (CMOS) micro-electro-mechanical-system (MEMS) thermal wall shear stress sensor based on a tungsten hot-wire and a single thermopile. Devices were fabricated using a commercial 1 μm SOI-CMOS process followed by a deep reactive ion etching (DRIE) back-etch step to release a silicon dioxide membrane, which mechanically support and thermally isolates heating and sensing elements. The sensors show an electro-thermal transduction efficiency of 50 µW/°C, and very small zero flow off-set. Calibration for wall shear stress measurement in air in the range of 0 - 0.48 Pa was performed using a suction type, 2-D flow wind tunnel. The sensors were found to be extremely sensitive, up to 4 V/Pa for low wall shear stress values. Furthermore, we demonstrate the superior signal to noise ratio (up to five times higher) of a single thermopile readout configuration compared with a double thermopile readout configuration (embedded for comparison purposes within the same device). Finally, we verify that the output of the sensor is proportional to the cube root of the wall shear stress and we propose an accurate semi-empirical formula for its modelling.
    Full-text · Article · Jun 2015 · IEEE Sensors Journal
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    ABSTRACT: This paper reports on the novel deposition of zinc oxide (ZnO) nanorods using dip pen nanolithographic (DPN) technique on SOI (silicon on insulator) CMOS MEMS (micro electro mechanical system) micro-hotplates (MHP) and their characaterisation as a low-cost, low-power ethanol sensor. The ZnO nanorods were synthesized hydrothermally and deposited on the MHP that comprises a tungsten micro-heater embedded in a dielectric membrane with gold interdigitated electrodes (IDEs) on top of an oxide passivation layer. The micro-heater and IDEs were used to heat up the sensing layer and measure its resistance, respectively. The sensor device is extremely power efficient because of the thin SOI membrane. The electro-thermal efficiency of the MHP was found to be 8.2°C/mW, which results in only 42.7 mW power at an operating temperature of 350°C. The CMOS MHP devices with ZnO nanorods were exposed to PPM levels of ethanol in humid air. The sensitivity achieved from the sensor was found to be 5.8%/ppm to 0.39%/ppm for the ethanol concentration range 25 – 1000 ppm. The ZnO nanorods showed optimum response at 350°C. The CMOS sensor was found to have a humidity dependence that needs consideration in realworld application. The sensors were also found to be selective towards ethanol when tested in presence of toluene and acetone. We believe that the integration of ZnO nanorods using DPN lithography with a CMOS MEMS substrate offers a low cost, low power, smart ethanol sensor that could be exploited in consumer electronics.
    No preview · Article · May 2015 · RSC Advances
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    ABSTRACT: In this letter, we present a fully complementary-metal-oxide-semiconductor (CMOS) compatible microelectromechanical system thermopile infrared (IR) detector employing vertically aligned multi-walled carbon nanotubes (CNT) as an advanced nano-engineered radiation absorbing material. The detector was fabricated using a commercial silicon-on-insulator (SOI) process with tungsten metallization, comprising a silicon thermopile and a tungsten resistive micro-heater, both embedded within a dielectric membrane formed by a deep-reactive ion etch following CMOS processing. In-situ CNT growth on the device was achieved by direct thermal chemical vapour deposition using the integrated micro-heater as a micro-reactor. The growth of the CNT absorption layer was verified through scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The functional effects of the nanostructured ad-layer were assessed by comparing CNT-coated thermopiles to uncoated thermopiles. Fourier transform IR spectroscopy showed that the radiation absorbing properties of the CNT adlayer significantly enhanced the absorptivity, compared with the uncoated thermopile, across the IR spectrum (3 μm–15.5 μm). This led to a four-fold amplification of the detected infrared signal (4.26 μm) in a CO2 non-dispersive-IR gas sensor system. The presence of the CNT layer was shown not to degrade the robustness of the uncoated devices, whilst the 50% modulation depth of the detector was only marginally reduced by 1.5 Hz. Moreover, we find that the 50% normalized absorption angular profile is subsequently more collimated by 8°. Our results demonstrate the viability of a CNT-based SOI CMOS IR sensor for low cost air quality monitoring.
    Full-text · Article · May 2015 · Applied Physics Letters
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    ABSTRACT: In this letter, we present a fully complementary-metal-oxide-semiconductor (CMOS) compatible microelectromechanical system thermopile infrared (IR) detector employing vertically aligned multi-walled carbon nanotubes (CNT) as an advanced nano-engineered radiation absorbing material. The detector was fabricated using a commercial silicon-on-insulator (SOI) process with tungsten metallization, comprising a silicon thermopile and a tungsten resistive micro-heater, both embedded within a dielectric membrane formed by a deep-reactive ion etch following CMOS processing. In-situ CNT growth on the device was achieved by direct thermal chemical vapour deposition using the integrated micro-heater as a micro-reactor. The growth of the CNT absorption layer was verified through scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The functional effects of the nanostructured ad-layer were assessed by comparing CNT-coated thermopiles to uncoated thermopiles. Fourier transform IR spectroscopy showed that the radiation absorbing properties of the CNT adlayer significantly enhanced the absorptivity, compared with the uncoated thermopile, across the IR spectrum (3 μm–15.5 μm). This led to a four-fold amplification of the detected infrared signal (4.26 μm) in a CO2 non-dispersive-IR gas sensor system. The presence of the CNT layer was shown not to degrade the robustness of the uncoated devices, whilst the 50% modulation depth of the detector was only marginally reduced by 1.5 Hz. Moreover, we find that the 50% normalized absorption angular profile is subsequently more collimated by 8°. Our results demonstrate the viability of a CNT-based SOI CMOS IR sensor for low cost air quality monitoring.
    Full-text · Article · May 2015 · Applied Physics Letters
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    ABSTRACT: A new IGBT type structure, namely the p-ring FS+ Trench IGBT, with improved performance has been demonstrated. The improvement has been achieved through the utilization of p doped buried layers (p-rings) which allows for the simultaneous increase in the n enhancement layer doping concentration above the conventional levels without compromising the device breakdown rating. This unique lateral charge compensation approach is demonstrated to be highly effective in lowering the on-state losses. The experimental results show a 20% reduction in the on-state losses for a 1.7kV device.
    No preview · Conference Paper · May 2015
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    ABSTRACT: In this work we present the first experimental results of a Bi-mode Gate Commutated Thyristor (BGCT). The BGCT is a new type of Reverse Conducting-Integrated Gate Commutated Thyristor (RC-IGCT). In a conventional RC-IGCT, the IGCT and diode are integrated into a single wafer but they are fully separated from each other. The novel BGCT on the other hand features an interdigitated integration of diode- and GCT-areas. This interdigitated integration results in an improved diode as well as GCT area, better thermal distribution, soft turn-off/reverse recovery and lower leakage current compared to conventional RC-IGCTs. We have discussed the advantages of a new diode anode design in BGCT, which is shallower than that of the conventional RC-IGCT. We have successfully demonstrated the BGCT concept with 38 mm, 4.5 kV prototypes and compared the on-state, turn-off and blocking characteristics with conventional RC-IGCTs both in GCT- and diode-modes of operation.
    No preview · Conference Paper · May 2015
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    ABSTRACT: Most of the variables measured in scientific investigations or engineering applications depend, by varying degrees, on temperature. This necessitates the simultaneous measurement of temperature along with the variable of interest in order to perform high fidelity temperature compensated measurements. Silicon diode based temperature sensors (or silicon thermodiodes) have the advantages of being low cost, having an absolute temperature measurement capability as well as providing the option of on-chip integration with electronics circuits and a wide temperature measurement range. Leveraging these advantages, engineers and scientists have used silicon thermodiodes in numerous and diverse applications. This paper identifies the common temperature measuring techniques, and focuses on the use and advantages offered by silicon diodes operated as temperature sensors in different drive modes. Finally it explores the published literature for summarizing the application areas where such sensors have been utilized successfully in recent years.
    Full-text · Article · May 2015 · Sensors and Actuators A Physical
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    Full-text · Dataset · Apr 2015

Publication Stats

2k Citations
271.22 Total Impact Points

Institutions

  • 1994-2015
    • University of Cambridge
      • Department of Engineering
      Cambridge, England, United Kingdom
  • 2010
    • The University of Warwick
      • School of Engineering
      Coventry, England, United Kingdom
  • 2004
    • University of Toronto
      Toronto, Ontario, Canada
    • Valahia University of Târgoviste
      Tîrgovişte, Dâmboviţa, Romania