Search

An interesting Pd/Al 0.24 Ga 0.76 As-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 980 ppm H 2 /air environment, the concentration of hydrogen adsorption sites available at the metal–semiconductor interface, n i , and the effective distance, d , from the Pd–AlGaAs interface to adsorbed hydrogen atoms are 9.5 × 10 13 cm −2 and 3 A, 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.

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 N 2 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 I DS operating regime (>0.8 g m,max ) are 115.3 (115.1) and 108.2 (106.2) mA/mm in air (N 2 ) and 1% H 2 /air (1% H 2 /N 2 ), respectively. The variation trend of g m 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 ( I on / I off ) towards hydrogen gas is attributed to the considerable variation of I off in the cut-off region.

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 H 2 /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.

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 ?, respectively. The negative temperature dependence on Sr is due to the lower hydrogen coverage at higher temperature. Yet, the positive temperature dependence on ? 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.

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 4.3 ppm H2/N2 (98 ppm H2/air) at 130 °C, a high sensitivity of 1295 μA/mm-ppm H2/N2 (275.8 μA/mm-ppm H2/air) in 14 ppm H2/N2 (H2/air) at 30 °C, a fast transient response time of 2 (3) s in 9970 ppm H2/N2 (H2/air) at 130 °C, and a large initial rate of 774.4 (589.8) μA/s in 9970 ppm 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.

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 ( S R = 100 × ( R air − R H 2 ) / R air ) 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.

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.

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 H2, 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 H2 /air) and higher sensitivity (24.7% in 9970 ppm H2/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

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.

An interesting Pd–GaAs high electron mobility transistor (HEMT) hydrogen sensor is fabricated and studied. For the studied device, a 5 nm-thick undoped GaAs cap layer is grown to suppress the oxidation of the underneath Al 0.24 Ga 0.76 As layer. Comprehensive analysis on the electrical properties including equilibrium adsorption (steady-state) and kinetic adsorption (transient) is presented. Experimentally, a high current variation of 17.1 mA/mm is obtained in 9970 ppm H 2 /air gas at 323 K. A high channel conductance variation of 25.1 mS/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 −112.74 kJ mol −1 and −367.39 J mol −1 K −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 33.09 kJ mol −1 is obtained.

The interesting hydrogen sensing characteristics of two transistors with an Al 0.24 Ga 0.76 As (device A) and In 0.49 Ga 0.51 P (device B) Schottky layer are demonstrated and studied. Experimentally, device A shows a lower hydrogen detection limit of 4.3 ppm H 2 /air, a higher current variation of 7.79 mA and a shorter adsorption time of 10.95 s in a 9970 ppm H 2 /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 H 2 /air the hydrogen sensing properties of device B can be obtained as the temperature increases from 30 to 160 °C. Because the Al 0.24 Ga 0.76 As and In 0.49 Ga 0.51 P 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.

On the basis of a Pt/In0.52Al0.48As metal-semiconductor structure, a novel hydrogen sensor is fabricated and demonstrated. The studied Pt/In0.52Al0.48As Schottky diode-type hydrogen sensor exhibits significant sensing performance including high relative sensitivity ratio of about 2600% (under the 1% H2/air gas and VR=-0.5 V at 30 degC), large current variation of 310 muA (under the 1% H2/air gas and VR=-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% H2/air gas at 30 degC). The hydrogen concentrations and operating temperatures tested in this letter are in the range of 15 ppm-1% H2/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

An interesting two-step passivation (with ledge structure and sulphide based chemical treatment) on base surface, for the first time, is demonstrated to study the temperature-dependent DC characteristics and noise performance of an InGaP/GaAs heterojunction bipolar transistor (HBT). Improved transistor behaviors on maximum current gain βmax, offset voltage ΔVCE, and emitter size effect are obtained by using the two-step passivation. Moreover, the device with the two-step passivation exhibits relatively temperature-independent and improved thermal stable performances as the temperature is increased. Therefore, the two-step passivationed device can be used for high-temperature and low-power electronics applications.

An interesting hydrogen sensor based on an Al/sub 0.24/Ga/sub 0.76/As Schottky barrier high-electron mobility transistor with a catalytic Pd metal/oxide/semiconductor is fabricated and demonstrated. In comparison with traditional Schottky diodes or capacitance-voltage type hydrogen sensors, the studied device exhibits larger current variation, lower hydrogen detection limit, and shorter transient hydrogen response time. Besides, good hydrogen-sensing properties, such as significant drain current change, threshold voltage shift, and transconductance change of transistor behaviors, are obtained. Therefore, the studied device provides the promise for high-performance solid-state hydrogen sensors, optoelectronic integrated circuits, and microelectromechanical system applications.

The temperature-dependent characteristics of an InGaP/InGaAs/GaAs heterostructure field-effect transistor (HFET), using the (NH/sub 4/)/sub 2/S/sub x/ solution to form the InGaP surface passivation, are studied and demonstrated. The sulfur-passivated device shows significantly improved dc and RF performances over a wide temperature range (300-510 K). With a 1/spl times/100-/spl mu/m/sup 2/ gate-dimension HFET by (NH/sub 4/)/sub 2/S/sub x/ treatment, the considerably improved thermal stability over dc performances including lower temperature variation coefficients on the turn-on voltage (-1.23 mV/K), the gate-drain breakdown voltage (-0.05 mV/K), the gate leakage current (1.04 /spl mu/A/mm/spl middot/K), the threshold voltage (-1.139 mV/K), and the drain-saturation-current operating regimes (-3.11/spl times/10/sup -4//K) are obtained as the temperature is increased from 300 to 510 K. In addition, for RF characteristics, the sulfur-passivated device also shows a low degradation rate on drain-saturation-current operating regimes (-3.29/spl times/10/sup -4//K) as the temperature is increased from 300 to 400 K. These advantages provide the promise for high-speed high-frequency high-temperature electronics applications.

By combining the advantages of a catalytic metal Pd with a high-performance AlGaAs/InGaAs/GaAs pseudomorphic high-electron-mobility transistor (PHEMT), an interesting hydrogen sensor is fabricated and demonstrated. For the proposed device, a 50 A undoped GaAs cap layer is grown to prevent the Al0.24Ga0.76As Schottky layer from oxidizing and to reduce the Fermi level pinning effect. Experimentally, a high sensitivity SJ value of 275.8 µA/mmppm H2/air can be obtained at a hydrogen concentration of 14 ppm H2/air. Even at a very low hydrogen concentration (≤4.3 ppm H2/air) at 30°C, a significant current variation can be observed. In addition, a fast transient response is found. The adsorption time constant τa becomes only 2 s as the operating temperature is elevated to 160°C. Therefore, the proposed device reveals the promise for high-performance hydrogen sensor applications.

An interesting transistor-type hydrogen sensing detector based on a GaAs pseudomorphic high electron mobility transistor (PHEMT) with a Pd/Al0.24Ga0.76As metal–semiconductor Schottky gate structure is fabricated and investigated. Steady-state properties and transient responses under different temperatures and hydrogen concentrations are measured and studied. Significant modulations in electrical signals are observed, obviously due to the adsorption of hydrogen atoms at the Pd–semiconductor interface. Also, the studied device exhibits fast response and recovery properties. The corresponding adsorption and desorption time constants (τa and τb) are 2.5 and 6 s, respectively, under 9970 ppm H2/air gas at 160 ◦ C. Furthermore, based on the Langmuir isotherm and the van’t Hoff equation, a hydrogen adsorption heat of −37.02 kJ mole −1 is obtained at lower operating temperatures (72 ◦ C). However, at a high temperature

The influences of (NH/sub 4/)/sub 2/S/sub x/ treatment on an AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistor (PHEMT) are studied and demonstrated. Upon the sulfur passivation, the studied device exhibits better temperature-dependent dc and microwave characteristics. Experimentally, for a 1/spl times/100 /spl mu/m/sup 2/ gate/dimension PHEMT with sulfur passivation, the higher gate/drain breakdown voltage of 36.4 (21.5) V, higher turn-on voltage of 0.994 (0.69) V, lower gate leakage current of 0.6 (571) /spl mu/A/mm at V/sub GD/=-22 V, improved threshold voltage of -1.62 (-1.71) V, higher maximum transconductance of 240 (211) mS/mm with 348 (242) mA/mm broad operating regime (>0.9g/sub m,max/), and lower output conductance of 0.51 (0.53) mS/mm are obtained, respectively, at 300 (510) K. The corresponding unity current gain cutoff frequency f/sub T/ (maximum oscillation frequency f/sub max/) are 22.2 (87.9) and 19.5 (59.3) GHz at 250 and 400 K, respectively, with considerably broad operating regimes (>0.8f/sub T/,f/sub max/) larger than 455 mA/mm. Moreover, the relatively lower variations of device performances over wide temperature range (300/spl sim/510 K) are observed.

An interesting hydrogen sensor based on a high electron mobility transistor (HEMT) device with a Pd–oxide–In0.49Ga0.51P gate structure is fabricated and demonstrated. The hydrogen sensing characteristics including hydrogen detection sensitivity and transient responses of the studied device under different hydrogen concentrations and temperature are measured and studied. The hydrogen detection sensitivity is related to a change in the contact potential at the Pd/insulator interface. The kinetic and thermodynamic properties of hydrogen adsorption are also studied. Experimentally, good hydrogen detection sensitivities, large magnitude of current variations (3.96 mA in 9970 ppm H2/air gas at room temperature) and shorter absorption response time (22 s in 9970 ppm H2/air gas at room temperature) are obtained for a 1.4 μm × 100 μm gate dimension device. Therefore, the studied device provides a promise for high-performance solid-state hydrogen sensor, integrated circuit (IC) and micro electro-mechanical system (MEMS) applications.

An interesting hydrogen sensor based on a Pt–oxide–Al0.3Ga0.7As MOS structure is fabricated and studied. The transient response and hydrogen detection sensitivity of the studied device under different hydrogen concentrations, temperature and applied voltages are measured. In addition, the kinetic and thermodynamic properties of hydrogen adsorption are studied and discussed. Experimentally, the studied device shows significant advantages of extremely low hydrogen concentration (15 ppm H2/air) detection capability, high hydrogen detection sensitivity (20 in 9090 ppm H2/air gas), short response time, wide temperature operating regime and good hydrogen detection ability under bi-directional applied bias. Based on these good properties, therefore, the studied device provides a promise for high-performance solid-state hydrogen sensor applications.

Pd- and Pt-Al0.3Ga0.7As metal–semiconductor (MS) Schottky diodes are fabricated and studied. The hydrogen-sensing characteristics including Schottky barrier height modulations and hydrogen detection sensitivity and transient responses are investigated and presented. Due to the formation of hydroxyl at higher temperature, different transient responses are found for the studied Pt- and Pd-MS Schottky diodes. According to the van't Hoff equation, the initial heat of adsorption for the Pd- and Pt-Al0.3Ga0.7As MS interfaces is calculated as −5.21 and −7.56 kJ mole−1, respectively.

The hydrogen response characteristics and sensing properties or catalytic Pd/Al/sub 0.3/Ga/sub 0.7/As metal-oxide-semiconductor (MOS) and metal-semiconductor (MS) Schottky diodes are systematically studied. The effects of hydrogen adsorption on device performances such as the current-voltage characteristics, sensitivity, barrier height variation, heat of adsorption, and transient response are investigated. The studied devices can be operated under very wide hydrogen concentration regimes with remarkable hydrogen-sensing properties. Particularly, at an extremely low hydrogen concentration of 15 ppm H/sub 2//air, both steady-state and transient responses at room temperature can be detected. In addition, under the presence of oxide layer in the studied MOS device, a larger change of barrier height and higher hydrogen response are observed. In addition, according to the van't Hoff equation, the initial values of heat adsorption for Pd/semiconductor and Pd/oxide interface are calculated as 7.29 and 49.6 KJ/mole, respectively.

An interesting hydrogen sensing Pd/InGaP metal-semiconductor (MS) Schottky diode has been fabricated and studied. Both the steady state and the transient condition of the hydrogen adsorption process are investigated. Even at room temperature, an extremely low hydrogen concentration of 15 ppm H2/air can be detected. In addition, the wide operating temperature range of 250 K of the studied Pd/InGaP hydrogen sensor is found. From experimental results, it is shown that the variation of Schottky barrier height increases with the increase of the operating temperature and hydrogen concentration. As the operation temperature is elevated, the water formation effect is also studied in the quasi-equilibrium region under the transient condition.

The emitter ledge length effect on the performance of InGaP/GaAs heterojunction bipolar transistors is comprehensively studied. It is shown that the undesired potential saddle point at the edge of emitter ledge is substantially presented. At the saddle point, the electron density and recombination rate are decreased with increasing the emitter ledge length. However, the longer emitter ledge length increases the base-collector junction area which in turn deteriorates the high frequency performance. Therefore, the length of emitter ledge should be carefully considered. Based on the theoretical analysis and experimental results, the optimum emitter ledge length is near 0.8 µm.

A new and interesting field-effect resistive hydrogen sensor that is based on a Pd/oxide/AlGaAs pseudomorphic high electron mobility transistor (PHEMT) structure, is fabricated and studied. A simple model is proposed to elucidate the hydrogen adsorption and sensing mechanisms. The reaction of the device studied with hydrogen gas results in a considerable decrease in channel resistance. The resistance curves of the hydrogen response demonstrate that this device shows good reversible, repeatable, and concentration-dependent properties. In comparison with other resistor-type hydrogen sensors, the device studied presents the significant advantages of high detection sensitivity (24.7% in 9970 ppm H2/air gas) and low detection limit (< 4.3 ppm H2/air) at room temperature. Furthermore, the device studied exhibits small resistance (30 Ω) and small operating voltage (< 0.3 V), which are superior to those of other resistive sensors with typically larger resistances ranging from kΩ to MΩ and voltages greater than 1 V.

CIGS solar cell has yielded the high efficiency beyond 20% on glass substrate [1], but some drawbacks such as heavy, not portable, toxic and costly need to be met for future manufacture. In this study, ZnS(O,OH)/CIGSeS solar cells were fabricated on flexible stainless steel substrate by low-cost nano-particle printing process. The flexible and non-toxic CIGSeS solar cell shows the efficiency as high as 13.37% without anti-reflection layer. The high efficiency was attributed to good thin film quality in absorption layer and buffer layer. Besides, the ZnS(O,OH) buffer layer with thickness only 15 nm shows good coverage, which enhances photocurrent and leads to high efficiency. Moreover, the CIGSeS solar cells were placed to dark room for 3 days to test light soaking effects, the results show that only 0.14% efficiency decay was measured. The results implies that the non-toxic and flexible CIGSeS solar cells prepared by nanoparticle printing process shows high potential for portable power source and building integrated photovoltaic system.

A new Pt-InAlP metal-oxide-semiconductor (MOS) Schottky diode hydrogen sensor with high-sensitive hydrogen detection among wide operating temperature regime is demonstrated and studied. Experimentally, the studied hydrogen sensor can be operated systematically either under forward or reverse bias conditions. At 30degC, the relatively hydrogen detection ratio Sr value is increased from 108.8% (107.4%) to 943.1% (1337.3%), under the forward (reverse) bias of 0.3 V, when the hydrogen concentration is increased from 4.3 to 9970 ppm H2/air. In addition, it is worth to note that even an extremely low hydrogen concentration of 4.3 ppm H2/air can be effectively detected at the temperature of 30~250degC.

A new and interesting compound semiconductor Schottky diode hydrogen sensor based on a Pt-In0.5Al0.5P metal-semiconductor (MS) structure is fabricated and demonstrated. The hydrogen sensing characteristics including hydrogen detection sensitivity of the studied device under different hydrogen concentrations and temperature are measured and studied. The hydrogen detection sensitivity is related to a change in the contact potential at the Pt-semiconductor interface. Experimentally, the studied hydrogen sensor can be operated systematically under the applied bi-polarity voltage biases. When the temperature is increased from 30 to 250degC, the hydrogen relative sensitivity ratio (Sr), under the applied forward (reverse) bias of 0.3 V, is decreased from 181.2% (250.3%) to 28.2% (33.5%) upon exposing to the 9970 ppm H2/air gas. Moreover, the hydrogen effect in the Schottky barrier height lowering is observed.

In this study, a triangular-barrier and a double-barrier structure were integrated to form a bi-directional switching device. In the structure center of the triangular barrier, a delta-doped (/spl delta/-doped) quantum well was inserted to enhance the carrier accumulation. Owing to the resonant tunneling through the double barrier and avalanche multiplication in the reverse-biased junction, N-shaped and S-shaped negative-differential-resistance (NDR) phenomena would occur in the current-voltage (I-V) characteristics under normal and reverse operation modes, respectively.

The temperature-dependent characteristics of polysilicon and diffused resistors have been studied. By using the 0.18 /spl mu/m CMOS technology, cobalt salicide process is employed and silicide is formed at the ends of resistors. Based on a simple and useful model, some important parameters of resistors including bulk sheet resistance (R/sub bulk/) and interface resistance (R/sub interface/) are obtained at different temperature. For diffused resistors, the R/sub bulk/ and R/sub interface/, values are increased and decreased with the increase of temperature, respectively. Positive values of temperature coefficient of resistance (TCR) are observed. Furthermore, TCR values are decreased with the decrease of resistor size. For polysilicon resistors, the Rinterface values are decreased with the increase of temperature. In addition, negative and positive TCR values of RNA are found in n/sup +/ and p/sup +/ polysilicon resistors, respectively. In conclusion, by comparing the studied diffused and polysilicon resistors, the negative trends of TCR are observed when the resistor sizes are decreased.

An interesting InGaP/InGaAs heterostructure field-effect transistor utilizing dual /spl delta/-doped quantum wells as double channels is studied and demonstrated. The employed dual /spl delta/-doped quantum wells and InGaP layer provide good carrier confinement and Schottky behavior, respectively. Good device performances including higher turn-on and breakdown voltages, high and linear transconductance and RF properties are obtained. For a 1 /spl times/ 100 /spl mu/m device, turn-on voltage of 1.74 V, maximum output current of 499 mA/mm, and maximum transconductance of 162 mS/mm with 303 mA/mm broad operation regime are obtained. The microwave properties of f/sub T/ and f/sub max/ are 16 and 32.3 GHz, respectively. Furthermore, even the device is operated at higher temperature regime (>400K), insignificant degradations of DC and RF performances are observed.

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 H2/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 Von and series resistance Rs values, respectively, is obtained at 30 °C, even at an extremely low hydrogen concentration of 4.3 ppm H2/N2. 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.

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 9.5×1013cm−2 and 3A, respectively. Furthermore, the simulated curves are in excellently agreement with the experimental results.

The influence of emitter ledge length on the performance of InGaP∕GaAs heterojunction bipolar transistors is comprehensively investigated. Due to the band-bending effect at the intersection of the emitter ledge edge with the exposed base surface, an undesired potential saddle point is formed. Moreover, emitter ledge passivations that are longer or shorter than an optimal length result in the deterioration of device performance. Based on the theoretical analysis and experimental results, the surface recombination effect of the device with an emitter ledge length of 0.8μm is negligible compared with the unpassivated device. Also, the device with the emitter ledge length of 0.8μm shows nearly the best dc characteristics and acceptable rf performance. Therefore, the optimum emitter ledge length in this study is near 0.8μm.

bChung Shan Institute of Science and Technology, Lung-Tan, Tao-Yuan, Taiwan 32544 Temperature-dependent characteristics of high electron mobility transistors HEMTs with sulfur and SiNx passivations are comprehensively 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 SiNx layer and presence of surface traps at the SiNx/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.

An interesting compound semiconductor Schottky diode hydrogen sensor based on a Pt-In 0.5 Al 0.5 P metal-semiconductor structure is fabricated and demonstrated. The hydrogen sensing characteristics, including hydrogen detection sensitivity and transient responses of the studied device under different hydrogen concentrations and temperatures, were measured and studied. The hydrogen detection sensitivity is related to a change in the contact potential at the Pi-semiconductor interface. Experimentally, the studied hydrogen sensor can be operated systematically under the applied bipolarity voltage biases. When the temperature is increased from 30 to 250°C, the hydrogen relative sensitivity ratio (S r ), under the applied forward (reverse) bias of 0.3 V, is decreased from 181.2% (250.3%) to 28.2% (33.5%) upon exposing to the 9970 ppm H 2 /air gas. Moreover, the hydrogen effect in the Schottky barrier height lowering is observed. The kinetic and thermodynamic properties of hydrogen adsorption were also studied.

In this work, the characteristics of the InGaP/GaAs 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 200 A, 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.

The thermal stability performance of double 8-doped In 0.42 Al 0.58 As/In 0.46 Ga 0.54 As metamorphic heterostructure field-effect transistors 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 X 10 -3 μA/mm K), output conductance (1.23 μS/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.

Temperature-dependent characteristics of a formal-passivated pseudomorphic high electron mobility transistor (PHEMT) are studied and demonstrated. Experimentally, for the PHEMT's operation, significant improvements of f T , f max , g m and I DS operating regime are found after formal passivation. In addition, remarkably lower temperature variation coefficients on V on (-1.33 mV/K), BV GD (-65.6 mV/K), Φ B (-0.321 meV/K) and n (3.88 x 10 -4 /K) are observed as temperature is increased from 300 to 480 K. Under an accelerated stress test, this formal-passivated device also shows improved reliability performance.

Comprehensive studies of temperature-dependent gate-metal-related impact ionization in metamorphic high electron mobility transistors (MHEMTs) are demonstrated. It is known that, from experimental results, the electric field and temperature dependences of impact ionization mechanisms in MHEMT’s operation are dominated by the ionization threshold energy and hot electron population. The peak impact ionization-induced gate current could be substantially decreased by the presence of specific gate contact with a higher Schottky barrier height. Therefore, the suppressions of bell-shaped behavior and related impact ionization effect are observed by using the appropriate Schottky gate contacts.

A novel and interesting three-terminal-controlled active resistor-type hydrogen sensor, based on good properties in the linear region of an AlGaAs-based pseudomorphic high electron mobility transistor in combination with the catalytic Pd metal, is demonstrated. The experimental results show that the gate–source voltage VGS exhibits significant influence on the hydrogen-sensing properties, including resistance sensitivity, detection limit of hydrogen concentration, conductance variation, current variation and dynamic response. Consequently, under an appropriate applied VGS bias, a smart active resistor-type hydrogen sensor can be achieved.

A new Pt-InAlP metal oxide semiconductor Schottky diode hydrogen sensor with highly sensitive hydrogen detection among wide operating temperature regime is demonstrated and studied. Experimentally, the studied hydrogen sensor can be operated systematically either under forward or reverse bias conditions. At 30°C, the relatively hydrogen detection ratio S r value is increased from 108.8% (107.4%) to 943.1% (1337.3%), under the forward (reverse) bias of 0.3 V, when the hydrogen concentration is increased from 4.3 to 9970 ppm H 2 /air. Note that even an extremely low hydrogen concentration of 4.3 ppm H 2 /air can be effectively detected at the temperature of 30-250°C.

The temperature-dependent direct current dc characteristics and radio frequency performance of an InGaP/GaAs heterojunctionbipolar transistor with the composite passivations on base surface are studied and demonstrated. For comparison, the character-istics of other samples with different treatments on the base surfaces are also included in this work. The device with compositepassivations, i.e., the ledge structure and sulfur treatment, shows the best performance and related thermal stabilities on dc currentgain