[show abstract][hide abstract] ABSTRACT: The interesting hydrogen sensing properties of a Pt-oxide-GaN metal-oxide-semiconductor-type Schottky diode are comprehensively studied and demonstrated. In the hydrogen-containing environment, the shift in current-voltage curves and decrease in turn-on voltage are found to be caused by the lowering of Schottky barrier height. Also, the corresponding series resistance is decreased from 191.8 (in air) to 155.3 Ω (for a 9970 ppm H <sub>2</sub>/ air gas) at 30 ° C . As the carrier gas is replaced by a nitrogen gas, a significant variation of 0.32 V and 19.56 Ω in the turn-on voltage V<sub>on</sub> and series resistance R<sub>s</sub> values, respectively, is obtained at 30 ° C , even at an extremely low hydrogen concentration of 4.3 ppm H <sub>2</sub>/ N <sub>2</sub> . Since the oxygen atoms will be dissolved on the Pt metal surface and react with hydrogen atoms by the formation of hydroxyl and water, the number of adsorbed hydrogen atoms on the Pt surface is reduced. Moreover, the shorter response time constant and the larger initial rate of current density variation are found even at room temperature.
Journal of Applied Physics 08/2008; · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: The transient phenomenon of hydrogen-sensing of a Pd-oxide-InGaP metal-oxide-semiconductor field-effect transistor (MOSFET) is studied. In an environment of N2 carrier gas, the sensing signal is logarithmically proportional to the hydrogen concentration over a temperature range from 30 to 160°C. Due to a high activation energy needed for initiating the reverse hydrogen releasing process, the responsive current signal during hydrogen detection does not always go back to the baseline. Over the high temperature region, the recovering curve can be divided into three parts: (i) the initial, (ii) the accumulation, and (iii) the revival stages. Because the recombination process of hydrogen atoms is very slow, a large amount of desorbed hydrogen atoms are appeared and accumulated on the Pd metal surface. A long desorption time is observed. However, in the presence of oxygen, a high speed desorption phenomenon is observed. The hydrogen adsorption rate is also enhanced. In addition, the same shift of drain current baseline for two carrier gas systems (N2 and air) is found even at high temperature.
Sensors and Actuators B-chemical - SENSOR ACTUATOR B-CHEM. 01/2008; 134(2):750-754.
[show abstract][hide abstract] ABSTRACT: An interesting Pd/Al0.24Ga0.76As-based field-effect-transistor-type hydrogen detector is fabricated and studied. The corresponding electronic and hydrogen sensing properties are measured and investigated. Based on the measured results, a hydrogen sensing model is developed. Theoretically, the dipolar layer formed by the hydrogen atoms adsorbed at the Pd–AlGaAs interface can be considered as a two-dimensional layer. Under the 980ppm H2/air environment, the concentration of hydrogen adsorption sites available at the metal–semiconductor interface, ni, and the effective distance, d, from the Pd–AlGaAs interface to adsorbed hydrogen atoms are 9.5×1013cm−2 and 3Å, respectively. The simulated curves show excellent agreement with experimental results. In addition, an anomalous decrease phenomenon in transient response is observed which may be caused by the formation of hydroxyl species and water.
Sensors and Actuators B-chemical - SENSOR ACTUATOR B-CHEM. 01/2008; 133(1):128-134.
[show abstract][hide abstract] ABSTRACT: By combining the advantages of a catalytic Pt metal with an InAlAs material system, an interesting hydrogen sensor is fabricated and demonstrated. The Pt/InAlAs Schottky diode-type sensor exhibits high sensing performance toward hydrogen gas. A comparative study between forward and reverse biases is presented. A simple detection model is proposed to elucidate the hydrogen sensing behavior under forward and reverse biases. Thermionic emission (TE) and field emission (FE) exhibit considerable influences on the hydrogen sensing properties. Moreover, the temperature-dependent hydrogen detection characteristics are presented and studied. High sensor response is observed under reverse voltage, while large current variation is found under forward voltage. It is worth to note that this sensor shows a widespread reverse voltage-operating regime (0 to −5 V) with stable and flat sensing curves. The effective Schottky barrier height change and the series resistance variation, from the Norde plots, are −87.0 meV and −288 Ω, respectively, in 10,000 ppm H2/air at 303 K. Based on the significant advantage of integration compatibility with InP-based electronic devices, the studied device reveals the promise in smart sensor and micro-electro-mechanical system (MEMS) applications.
[show abstract][hide abstract] ABSTRACT: Based on a GaAs-based heterostructure field-effect transistor (HFET) equipped with a catalytic Pd gate, an interesting Pd/GaAs transistor-type hydrogen sensor is fabricated and studied. A simple model is used to elucidate the on-state and off-state behaviors in the transistor operation. In air and N2 environments, hydrogen-induced effect not only causes an obvious current variation in the saturation region, but also results in a drastic change of sensor response in the cut-off region. The further analyses of electrical characteristics are also presented. The calculated values of IDS operating regime (>0.8 gm,max) are 115.3 (115.1) and 108.2 (106.2) mA/mm in air (N2) and 1% H2/air (1% H2/N2), respectively. The variation trend of gm under the positive gate bias is contrary to that under the negative gate bias in hydrogen-containing ambiance. In addition, the decrease in on–off current ratio (Ion/Ioff) towards hydrogen gas is attributed to the considerable variation of Ioff in the cut-off region.
Sensors and Actuators B-chemical - SENSOR ACTUATOR B-CHEM. 01/2008; 132(2):587-592.
[show abstract][hide abstract] ABSTRACT: A new and interesting field-effect resistive hydrogen sensor, based on the current-voltage characteristics in the linear region of an AlGaAs-based pseudomorphic high-electron-mobility transistor structure and high hydrogen sensitivity of a palladium (Pd) metal, is studied and demonstrated. An oxide layer between Pd and AlGaAs is used to increase the number of hydrogen adsorption sites, and improve hydrogen detection sensitivity. A simple model is employed to interpret the hydrogen adsorption and sensing mechanism. The dissociation of H<sub>2</sub>, diffusion of H atoms and formation of a dipolar layer cause a significant decrease in channel resistance. In comparison with other resistor-type hydrogen sensors, the studied device demonstrates the considerable advantages of lower detection limit (< 4.3 ppm H<sub>2 </sub>/air) and higher sensitivity (24.7% in 9970 ppm H<sub>2</sub>/air) at room temperature. Also, the studied device exhibits a smaller resistance (several 10 Omega) and a smaller operating voltage (les 0.3 V) which are superior to other resistive sensors with typically larger resistances (ranged from kiloohms to megaohms) and larger voltages (ges 1 V). Consequentially, the studied resistive sensor provides the promise for low-power GaAs-based electronic and microelectromechanical-system applications
IEEE Transactions on Electron Devices 06/2007; · 2.06 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this work, the characteristics of the
heterojunction bipolar transistors with different emitter-edge-thinning thickness were systematically investigated. A stronger
downward-band-bending phenomenon was observed at the edge of emitter-edge-thinning intersection with the exposed base surface.
This band bending induced the presence of a potential saddle point, which substantially increased the recombination rates
and electron densities. In addition, the decision of emitter-edge-thinning thickness plays a key role in reducing surface
recombination at the potential saddle point. As the emitter-edge-thinning thickness was selected between 100 and
, the lowest recombination rate and electron density and highest dc current gain could be obtained. Furthermore, good agreements
between the theoretical analyses and experimental results were found.
Journal of The Electrochemical Society. 03/2007; 154(4):H289-H292.
[show abstract][hide abstract] ABSTRACT: Comprehensive and systematical comparisons of temperature-dependent characteristics of In0.42Al0.58As/In0.46Ga0.54As metamorphic heterostructure field-effect transistors (MHFETs) with various Schottky gate alloys are studied and demonstrated. The influence of the Schottky barrier height on the impact ionization effect and its associated device performance are also investigated. Better dc and microwave characteristics can be obtained by using the higher metal work function of gate alloys, e.g., Ti/Au, Ni/Au and Pt/Au. In particular, the device with a Pt/Au gate alloy shows the superior device performance in breakdown voltage, threshold voltage, maximum transconductance, output conductance, voltage gain and microwave properties at room temperature. Furthermore, the device with a Ti/Au gate alloy shows the thermally stable performance in threshold voltage, maximum transconductance, output conductance and voltage gain over a wide operating temperature range (from 300 to 510 K). Consequently, the studied devices with appropriate Schottky gate contacts provide the promise for high-speed and high-temperature electronic applications.
Semiconductor Science and Technology 03/2007; 22(5):475. · 1.92 Impact Factor
[show abstract][hide abstract] ABSTRACT: The thermal stability performance of double -doped In 0.42 Al 0.58 As/In 0.46 Ga 0.54 As metamorphic heterostructure field-effect tran-sistors with Au and Ti/Au metal gates are comprehensively studied and demonstrated. By evaporating the Ti/Au metal gate, the thermal stability of device characteristics are significantly improved as compared with the device with conventional metal gate Au. Experimentally, the device with a Ti/Au metal gate simultaneously exhibits the considerably lower temperature degradation in turn-on voltage −2.19 mV/K, breakdown voltage −34 mV/K, logic swing −1.24 mV/K, transition region width 0.05 mV/K, on-off current ratio −3.55 /K, threshold voltage −0.25 mV/K, impact ionization-induced gate current 1.63 10 −3 A/mm K, output conductance 1.23 S/mm K, and voltage gain −0.33 /K as the temperature is increased from 300 to 510 K. Consequently, the studied device with a Ti/Au metal gate is a good candidate for high-speed and high-temperature digital and switching circuit applications. Recently, metamorphic heterostructure field-effect transistors MHFETs have received much attention for high-performance inte-grated electronic circuit applications. 1,2 As compared with InP-based transistors, 3,4 the metamorphic structure offers several advantages such as low cost, less fragility, large substrate size, high crystal quality, and mature back-side processing. In addition, the metamor-phic structures give a free choice of lattice constant and indium composition of In x Ga 1−x As channel. However, the high In mole frac-tion of InGaAs channel used in MHFET is observed to easily initiate the impact ionizations. 5 If the electric field is high enough to accel-erate carriers within the channel, the electron gains kinetic energy to collide with the lattice and generate electron-hole pairs. 6 Some holes injected across the Schottky layer and trapped close to the gate terminal could lead to a negative shift of the threshold voltage. Subsequently, through the degraded gate control ability as well as reduced gate depletion region, the kink effect and bell-shaped be-havior occur. 2,7 Therefore, the deposition of mechanically stable metal gates has become a critical issue. In particular, the require-ments of a thermally stable Schottky gate contact are that the metal gates can survive during high-temperature process and device opera-tion. Because the frequently used Pt metal is easy to chemically react with GaAs or AlGaAs layer and forms compounds at relatively low temperature, 8 it may be unsuitable for application in the gate electrode of metal semiconductor field-effect transistors. In this work, the double -doped In 0.42 Al 0.58 As/In 0.46 Ga 0.54 As MHFETs with different metal gates, e.g., Ti/Au and Au, are fabri-cated to comprehensively study their thermal stability performance over a wide temperature range 300–510 K. Also, to study the elec-trical properties in digital and switching circuit applications, the voltage transfer characteristics are included. The temperature degra-dation rates in device characteristics depend strongly on the specific metal gates. By the employment of Ti as a part of metal gate, the lower deviation and higher performance of Schottky gate contacts are obtained even at high ambient temperature. This also implies that the impact ionizations and related thermal runaway are sup-pressed. Therefore, the higher temperature operation capability and relatively thermal stability are simultaneously enhanced in device operations.
[show abstract][hide abstract] ABSTRACT: Temperature-dependent characteristics of high electron mobility transistors HEMTs with sulfur and SiN x passivations are com-prehensively studied and demonstrated. Experimentally, for the studied device with formal passivations, better dc and microwave characteristics are obtained over a wide operating temperature range. In particular, as compared with the device only with sulfur passivation, the slight degradations of device performance are caused by the temperature stress during the deposition of SiN x layer and presence of surface traps at the SiN x /AlGaAs interface. However, under an accelerated stress test, this formal-passivated device shows improved reliability performance. Based on these good results, the formal-passivated HEMT is expected to exhibit relatively better long-term operation stability and reliable device characteristics. Recently, high electron mobility transistors HEMTs have been applied widely in high-performance microwave and power circuits. 1-3 It is known that, practically, the manufacturing cost, yield, device reliability, and longevity are mandatory factors for sys-tem applications. Because metal-semiconductor MS-based struc-tures are widely employed in III-V HEMTs, the surface effect in-duced by high density of interface traps at MS contacts has attracted much attention. The surface effects crucially influence device per-formance because the electron transport within channel layer is strongly affected by the air-exposed surface region. 4 Hence, the lack of an adequate surface passivation usually results in the formation of native oxide and degradation of electrical properties. To optimize device behaviors and achieve thermal stability, the sulfur passivation has been developing as a promising technique. 5-7 Previously, the sulfur passivation effects on the temperature-dependent characteristics of HEMTs with Pt/Au gate Schottky con-tact were demonstrated in our report. 7 However, the Pt metal is easy to chemically react with GaAs or AlGaAs layer at the relatively low temperature. 8 Thus, it is difficult to control the accuracy and preci-sion of sulfur passivation on the device with Pt/Au gate alloy. Also, the sulfur passivation effect could be degraded quickly when the device was exposed to ambient atmosphere. 9 This certainly leads to reliability problems for device performance. Therefore, deposition of an oxygen-free dielectric layer after sulfur passivation is neces-sary to protect the sulfide layer and achieve long-term stable passi-vation. Above all, silicon nitride SiN x is one of the dominant ma-terials for further stabilizing the sulfur passivation effect. 10,11 In this work, a formal passivation method that combines the sulfur and SiN x passivations is employed to achieve improved op-eration stability and reliable device performance in HEMTs with Ti/Pt/Au gate Schottky contact. To meet the desired accuracy, the Ti metal as a part of gate Schottky contact is employed. 12 This im-proved experimental condition would be useful to study the inherent effect of sulfur and SiN x passivations without any unpredictable effects. Moreover, the proposed passivation technique is substan-tially different from previous work. 7 For the formal passivation, an additional SiN x passivation layer is deposited on the mesa sidewall and the exposed sulfur-passivated barrier surface between drain-source and gate edge. The effects of sulfur and SiN x passivations in the electric parameters are analyzed and comprehensively studied. The comparisons between the formal-passivated device and nonpas-sivated device are made to show the effect of formal passivation on device performance. Furthermore, in order to demonstrate the supe-riority of the proposed formal passivation over standard sulfur pas-sivation, the sulfur-passivated device is also included in the reliabil-ity test.
Journal of The Electrochemical Society - J ELECTROCHEM SOC. 01/2007; 154(3).
[show abstract][hide abstract] ABSTRACT: Interesting hydrogen-sensing properties of catalytic Pt/In0.5Al0.5P metal-oxide–semiconductor (MOS) and metal–semiconductor (MS) Schottky diodes are comprehensively studied and compared. The effects of hydrogen adsorption are investigated on the device performance such as the current–voltage characteristics, relative sensitivity ratio, Schottky barrier height variation, and built-in electric field. Experimentally, both the hydrogen sensors can be operated systematically under bi-polarity biases. The detecting sensitivity of the MOS-type hydrogen sensor is superior to that of the MS-type. It is believed that a high-quality oxide layer effectively increases the amount of hydrogen atoms adsorbed. Also, the hydrogen effects are found on both the Schottky barrier height lowering and the modulation in the electric field at the Pt-oxide and Pt–InAlP interfaces. In addition, the influence of the oxygen partial pressure in synthetic air and the existence of an oxygen layer between the Pt metal and the InAlP material are also studied.
[show abstract][hide abstract] ABSTRACT: Based on a Pd/oxide/AlGaAs pseudomorphic high-electron-mobility transistor (PHEMT) structure, an interesting three-terminal-controlled field-effect resistive hydrogen sensor is fabricated and studied. The influences of gate-source bias (VGS) on the hydrogen sensing properties are presented in this work. Experimental results show that the VGS bias significantly affects the resistance sensitivity, conductance variation, current variation, transient response, pressure-dependent and -independent rate constants, and response and recovery time constants. At 30 °C, a significant resistance response (SR=100×(Rair−RH2)/Rair) of 33.3% (82.8%) to 4.3 (9970) ppm H2/air is obtained at VGS = −0.6 V. Nevertheless, the largest conductance variation (ΔG) appears to be in the range between VGS = −0.3 and −0.4 V. An empirical equation is derived to explain the consistency between the calculated data and experimental results. Good linear relationship is observed between current variation and temperature under different VGS biases. The transient response at VGS = −0.3 V shows larger current variations, accompanying the longer response and recovery time constants than those at VGS = 0 V. Furthermore, on the basis of a kinetic adsorption analysis, the hydrogen pressure-dependent and –independent rate constants are obtained.
[show abstract][hide abstract] ABSTRACT: A hydrogen sensor based on the Pd/GaAs pseudomorphic high electron mobility transistor (PHEMT) is fabricated and investigated under various hydrogen concentrations in air and N2 environments. Experimentally, in nitrogen (air) ambiances, the studied sensor exhibits a hydrogen detection limit of 4.3ppm H2/N2 (98ppm H2/air) at 130°C, a high sensitivity of 1295μA/mm-ppm H2/N2 (275.8μA/mm-ppm H2/air) in 14ppm H2/N2(H2/air) at 30°C, a fast transient response time of 2 (3) s in 9970ppm H2/N2 (H2/air) at 130°C, and a large initial rate of 774.4 (589.8)μA/s in 9970ppm H2/N2 (H2/air) at 90°C. However, the studied sensor shows a longer recovery time in nitrogen than in air due to the lack of additional desorption process. From the experimental results, it is speculated that the oxygen in air occupies the adsorption sites and reduces the adsorbed hydrogen atoms at the Pd/GaAs interface. This interprets that the studied sensor shows a larger hydrogen-induced current variation in nitrogen than in air under the same hydrogen concentration. In addition, there is no overshoot phenomenon in transient response observed in the nitrogen atmosphere due to the absence of oxygen.
Sensors and Actuators B-chemical - SENSOR ACTUATOR B-CHEM. 01/2007; 125(1):22-29.
[show abstract][hide abstract] ABSTRACT: The electric and hydrogen sensing properties of an interesting Pd-gate metal-semiconductor-type high electron mobility transistor are comprehensively studied. The dipolar layer formed by adsorbed hydrogen atoms at the semiconductor of Pd-AlGaAs interface is equivalent to a two-dimensional layer. The concentration of available hydrogen adsorption sites at the metal-semiconductor interface ni and the effective distance d from the Pd-AlGaAs interface to adsorbed hydrogen atoms are 9.5×1013 cm-2 and 3 A˚, respectively. Furthermore, the simulated curves are in excellently agreement with the experimental results.
[show abstract][hide abstract] ABSTRACT: On the basis of Pt/InAlAs metal-semiconductor (MS) structure, an interesting hydrogen sensor is fabricated. The Pt/InAlAs Schottky diode-type hydrogen sensor studied exhibits significant sensing performance including high relative sensitivity ratio (Sr) of about 2600%, widespread reverse voltage regime (0~ - 5 V), and stable hydrogen sensing current density-voltage (J-V) curves. The calculated Schottky barrier height change and series resistance variation from the thermionic emission model and Norde method are 87.0 meV and 288 Omega, respectively. The negative temperature dependence on Sr is due to the lower hydrogen coverage at higher temperature. Yet, the positive temperature dependence on Delta I is caused by thermal effect. Based on the excellent integration compatibility with InP-based electronic devices, the device studied does provide potential for high-performance sensor array applications.
[show abstract][hide abstract] ABSTRACT: The interesting hydrogen sensing characteristics of two transistors with an Al0.24Ga0.76As (device A) and In0.49Ga0.51P (device B) Schottky layer are demonstrated and studied. Experimentally, device A shows a lower hydrogen detection limit of 4.3 ppm H2/air, a higher current variation of 7.79 mA and a shorter adsorption time of 10.95 s in a 9970 ppm H2/air at room temperature. On the other hand, device B exhibits more stable hydrogen-sensing characteristics at high temperatures. Even at a low concentration of 14 ppm H2/air the hydrogen sensing properties of device B can be obtained as the temperature increases from 30 to 160 °C. Because the Al0.24Ga0.76As and In0.49Ga0.51P materials are lattice-matched to the GaAs substrate, the studied devices can be integrated as sensor arrays to obtain superior hydrogen sensing characteristics including higher sensing signals, lower detection limit, shorter response time, and widespread detection and temperature regimes.
[show abstract][hide abstract] ABSTRACT: An interesting Pd–GaAs high electron mobility transistor (HEMT) hydrogen sensor is fabricated and studied. For the studied device, a 5nm-thick undoped GaAs cap layer is grown to suppress the oxidation of the underneath Al0.24Ga0.76As layer. Comprehensive analysis on the electrical properties including equilibrium adsorption (steady-state) and kinetic adsorption (transient) is presented. Experimentally, a high current variation of 17.1mA/mm is obtained in 9970ppmH2/air gas at 323K. A high channel conductance variation of 25.1mS/mm is also found under the same conditions. This indicates that, in hydrogen-containing ambience, the channel resistance reduces in the linear region of transistor operation. The reaction enthalpy and entropy are −112.74kJmol−1 and −367.39Jmol−1K−1, respectively. This interprets that the hydrogen adsorption process is exothermic and the hydrogen atoms are more ordered when they are adsorbed in a dipolar layer at the metal–semiconductor interface. In the transient analysis, the rate constants of the studied device can be calculated. Then the activation energy of about 33.09kJmol−1 is obtained.
Sensors and Actuators B-chemical - SENSOR ACTUATOR B-CHEM. 01/2007; 122(1):81-88.
[show abstract][hide abstract] ABSTRACT: On the basis of a Pt/In<sub>0.52</sub>Al<sub>0.48</sub>As metal-semiconductor structure, a novel hydrogen sensor is fabricated and demonstrated. The studied Pt/In<sub>0.52</sub>Al<sub>0.48</sub>As Schottky diode-type hydrogen sensor exhibits significant sensing performance including high relative sensitivity ratio of about 2600% (under the 1% H<sub>2</sub>/air gas and V<sub>R</sub>=-0.5 V at 30 degC), large current variation of 310 muA (under the 1% H<sub>2</sub>/air gas and V<sub>R</sub>=-5 V at 200 degC), widespread reverse-voltage regime (0~-5 V), stable hydrogen-sensing current-voltage (I-V) curves, and fast transient response time of 1.5 s. The calculated Schottky barrier-height change and series-resistance variation, from the thermionic-emission model and Norde method, are 87.0 meV and 288 Omega, respectively (under the 1% H<sub>2</sub>/air gas at 30 degC). The hydrogen concentrations and operating temperatures tested in this letter are in the range of 15 ppm-1% H<sub>2</sub>/air and 30 degC-250 degC, respectively. Based on the excellent integration compatibility with InP-based electronic devices, the studied device provides the potentiality in high-performance sensor-array applications
IEEE Electron Device Letters 01/2007; · 2.79 Impact Factor