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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.

Conformal passivation on an InGaP/GaAs HBT with significant reduction in the base surface-recombination effect is demonstrated. Not only dc behaviors but also RF performances are remarkably improved compared with the conventional emitter-ledge structure. Based on the conformal passivation, i.e., the base surface is covered by the depleted InGaP ledge structure and sulfur ((NH4)2Sx ) treatment, lower base surface-recombination current density, lower specific contact resistance, lower sheet resistance, higher current gain, higher collector current, and higher maximum oscillation frequency are obtained

Temperature-dependent dc characteristics and RF performances of InGaP/GaAs heterojunction bipolar transistors with sulfur treatment are systematically studied. The base-surface-recombination current, specific contact resistance, and sheet resistance of the studied devices can be effectively reduced by sulfur treatment. Practically, long-time sulfur treatment is not appropriate. In this paper, the studied device with the sulfur treatment for 12-15 min is a good choice. Experimentally, the collector-emitter offset voltage DeltaVCE and dc current gain with sulfur treatment can be substantially reduced and increased, respectively, over the 300-K-400-K temperature range. Moreover, as the temperature is increased, the device with sulfur treatment exhibits temperature-independent or thermally stable performances. The devices with sulfur treatment also exhibit improved RF characteristics

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

A comprehensive study of emitter-ledge thickness of InGaP/GaAs heterojunction bipolar transistors (HBTs) has been undertaken. It is shown that the recombination rate and electron densities are drastically increased near the exposed base surface between the base contact and the emitter ledge. In contrast, the corresponding hole densities are decreased. If the emitter ledge is too thick, current will flow through the undepleted ledge, which increases the emitter-size effect. In contrast, if the emitter ledge is too thin, it may not effectively passivate the surface. Therefore, the thickness of the emitter ledge is a crucial issue and should be carefully considered. It is shown that, from simulated and experimental results, the optimum emitter-ledge thickness of InGaP/GaAs HBT is 100-200 Aring

In this paper, an interesting thermally stable In0.42Al0.58As/In0.46Ga0.54As metamorphic high electron mobility transistor (MHEMT) is fabricated and investigated. Good dc and RF characteristics are obtained by precisely depositing gold (Au) upon the In0.42Al0.58As barrier layer as the Schottky contact metal. For a MHEMT with gate dimensions of 1 × 100 µm2, high gate–drain breakdown voltage, high turn-on voltage, low gate leakage current density, high maximum transconductance with broad operating regime and low output conductance are obtained even at ambient temperatures up to 510 K (240 °C). The studied device also shows a very good microwave performance at room temperature. Moreover, the relatively low variations of the device performance are achieved over a wide temperature range (from 300 to 510 K). Therefore, the studied device has a good thermally stable performance that is suitable for high-speed and high-power electronic 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.

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.

A new camel-gate field effect transistor (CAMFET) with a composite channel structure has been fabricated and demonstrated. Due to the n+-InGaP/p+-InGaP/GaAs camel gate and InGaAs/GaAs composite channel structures employed, good device performance is observed. Experimentally, at room temperature, a gate-drain breakdown voltage over 15 V, maximum transconductance gm,max of 111.5 mS mm?1, voltage gain AV of 93.4, unity current gain cut-off frequency fT of 16.2 GHz and maximum oscillation frequency unity fmax of 24.2 GHz are obtained simultaneously for a 1 ? 100 ?m2 device. The studied device also shows good properties in a higher temperature regime. Moreover, the studied device exhibits relatively negligible temperature-dependent characteristics over the operating temperature range from 300 to 420 K. Therefore, the studied device provides promise for high-temperature and high-performance microwave electronic applications.

InGaP/InGaAs double doped channel heterostructure field-effect transistors (DDCHFETs) with δ-doped channels and uniformly doped channels are comprehensively studied and demonstrated. From the simulation results, based on a two-dimensional simulator of Atlas, the band diagrams, DC and RF characteristics of studied devices are compared and studied. The better drain current drivability, higher transconductance and microwave performances are obtained in the studied device with δ-doped channel. In addition, experimentally, the DDCHFET with δ-doped quantum well channel is fabricated successfully. Due to the employed InGaAs double δ-doped channel structure and Schottky behaviours of InGaP 'insulator', good DC properties including pinch-off and saturation characteristics, higher and linear transconductance, and good RF properties are obtained. Moreover, the experimental results are consistent with simulated data.

It is shown that the influence of heat flux on internal energy required for consistency with thermodynamics of a frequently employed theory of second sound in nonmetallic solids implies that there are cases in which sinusoidal perturbations in temperature at the boundary of a region in which there is an underlying steady flux of heat q 0 result in temperature fields showing long-wavelength interference patterns propagating in the direction of q 0 at high (supersonic) speed.

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.

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.

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.

The temperature-dependent dc characteristics of InGaP∕GaAs heterojunction bipolar transistors with and without full sulfur passivation are systematically studied and demonstrated. The studied device with full sulfur passivation shows lower specific contact resistance ρC, lower sheet resistance Rsh, lower collector-emitter offset voltage ΔVCE, and higher dc gain βF than devices without sulfur passivation. The device with full sulfur passivation also exhibits better rf performance. In addition, the studied device with full sulfur treatment shows lower temperature variation coefficients of ρC,Rsh,ΔVCE, and βF. Thus, the device with sulfur treatment presents relatively temperature-independent characteristics that can extend the transistor action to higher temperature regimes.

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.