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The authors report the growth of high-density ZnSe/CdSe multiquantum disks on oxidized Si substrate. It was found the as-grown nanotips were tapered with the mixture of cubic zinc blende and hexagonal wurtzite structures. Also, photoluminescence intensities observed from these ZnSe/CdSe multiquantum disks were much larger than that observed from the homogeneous ZnSe. Activation energies for the ZnSe/CdSe multiquantum disks with well widths Lw of 8, 12, and 16 nm were 22, 62, and 56 meV, respectively.

The hydrogen sensing properties of a Pd/InAlAs metamorphic high electron mobility transistor (MHEMT) are investigated. Experimentally, a threshold voltage shift (Δ V th ) of 260 mV is observed upon exposing to a 1% H 2 /air gas. The drain current sensing response ( S R ) shows the strong dependence on the gate bias voltage V GS . A maximum S R of 107% is found at the applied voltage of V GS = −0.5 V. In addition, the temperature behavior of S R is predominantly determined by the hydrogen sticking coefficient and the exothermic reaction of hydrogen adsorption. It is also found that rectification ratio R can be changed with different hydrogen concentrations Furthermore, the response rate analyses reveal that the initial response rate is increased with the hydrogen concentration and temperature. The activation energy E a of 2.88 kJ mol −1 suggests that the studied Pd/InAlAs MHEMT hydrogen sensor has a low barrier for the adsorption of hydrogen.

In this work, the comprehensive study of the hydrogen adsorption effects on the Pd/AlGaN-based MOS Schottky diode with SiO 2 passivation is demonstrated. Hydrogen sensing performance of the proposed device is significantly enhanced with the insertion of SiO 2 layer. The MOS diode exhibits a maximum sensing response value of 3.3 × 10 5 at 70 °C under a 1% H 2 /air gas. Additionally, the operating temperature for the maximum sensing response is decreased from 150 °C to 70 °C with the insertion of SiO 2 layer. Moreover, the lowering of Schottky barrier height is 230 (200) meV for the MOS (MS) diode at room temperature. The hydrogen adsorption effect also largely influence the effective Richardson's constant. Furthermore, the transient-state experiments reveal that the MOS Schottky diode exhibits an activation energy of 14.1 kJ/mol which is lower than the MS Schottky diode (25 kJ/mol). Therefore, hydrogen sensing performance of the AlGaN-based Schottky diode can be significantly enhanced with the SiO 2 passivation on the AlGaN surface.

An interesting Pd-AlGaN/GaN Schottky diode-type hydrogen sensor with ultrahigh sensing responses is fabricated and studied. A sensing response (SF) of 2.04 times 105 and a widespread Schottky barrier height variation (DeltaphiB) of 400 meV are observed upon exposure to a 9660 ppm H2/air gas at 150degC. The high sensing response can be attributed to the catalytic capability of Pd metal with contributions from a high-density 2-D electron gas induced from spontaneous and piezoelectric polarizations. From a Langmuir isotherm, the equilibrium constants are found to be 1.9 and 0.7 torr-1 at 150degC and 200degC, respectively. The fast response and recovery times (< 10 s) are found under a 9660 ppm H2/air gas. Consequently, the compatible process of the AlGaN/GaN sensor makes it possible to be integrated into other high-sensing performance sensor for microelectrical and mechanical applications.

An interesting InGaP/InGaAs double channel pseudomorphic high electron mobility transistor is fabricated and demonstrated. Due to the employed InGaP Schottky and buffer layers, InGaAs double channel structure, and graded triple delta-doped sheets, both DC and RF performances are improved. From experimental results, the studied device, with a gate dimension of 0.8 x 100 mum2 , shows a drain saturation current of 176 mA/mm, a maximum extrinsic transconductance of 176 mS/mm, a unity current gain cutoff frequency of 16 GHz, and a maximum oscillation frequency of 33.2 GHz at room temperature. Moreover, a theoretical analysis based on a 2-D semiconductor simulation package is used to study the device properties and compare the experimental results. Good agreement between the theoretical analyses and experimental results is found.

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.

An interesting InGaP/GaAs heterojunction bipolar transistor (HBT) with a two-step passivated (ledge structure and sulfur treatment) process on the base surface has been fabricated and studied. Based on the two-step passivation, improved transistor characteristics including the specific contact resistances ρ C , sheet resistances R sh , base surface recombination current density J SR , base current ideality factor n B , and microwave performances are obtained. Furthermore, the device with two-step passivation reveals the better thermal stability on ρ C , R sh , and n B than the devices with and without ledge structure. Therefore, the two-step passivation method can be employed for high-temperature and low-power electronics applications.

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.

An interesting InP/InGaAs double heterojunction bipolar transistor with a step-graded InAlGaAs layer at the base-collector (B-C) heterojunction is fabricated and studied. Simulated results reveal that the potential spike at the B-C heterointerface is completely eliminated. Experimentally, the operation regime is wider than 11 decades in magnitude of the collector current (Ic = 10-12 A to Ic = 10-1 A). Furthermore, the studied device exhibits a relatively high common-emitter breakdown voltage and low output conductance even at high temperature. In the microwave characteristics, the unity current gain cutoff frequency fT = 72.7 GHz and the maximum oscillation frequency f max = 50 GHz are achieved for a nonoptimized device (AE = 6 times 6 mum2).

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.

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.

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

The temperature-dependent dc characteristics and noise performance of an interesting InGaP/GaAs heterojunction bipolar transistor (HBT) with emitter ledge passivation are demonstrated. Experimentally, due to the emitter ledge passivation, higher current gains and wider collector current over the measured temperature range (300–400 K) are observed as compared to a conventional device. In addition, the studied device exhibits lower base current ideality factors, better thermal stabilities on dc current gains, lower base surface recombination current densities and improved device reliability. Therefore, the studied device is suitable for low-power and high-temperature electronic 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

The characteristics of AlGaAs/InGaAs/GaAs depletion-mode (D-mode) and enhancement-mode (E-mode) pseudomorphic high electron mobility transistors (PHEMTs) fabricated using an electroless-plated (EP) deposition approach are investigated. Under the low-temperature and low-energy conditions, the EP deposition approach can form a better metal-semiconductor interface. For the studied devices, with a 1times100 mum2 gate dimension, excellent characteristics of the maximum drain saturation current (168.9 mA/mm) and extrinsic transconductance (225.8 mS/mm) are obtained for the D-mode device. The corresponding values for the E-mode device are 152.5 mA/mm and 211.7 mS/mm, respectively. Moreover, the EP approach also has the advantages of easy operation and low cost.

Hydrogen sensing properties of a Pd/AlGaN-based Schottky diode are improved by the deposition of SiO2 at the metal/semiconductor (MS) interface. The wide Schottky barrier height variation of the MOS diode could be attributed to the large electric field across the SiO2 layer. This leads to the presence of more hydrogen dipoles caused by the polarization effect. The sensing response of the MOS diode at room temperature (1.3 × 105) is comparable to that of the MS one at 150 °C (2.04 × 105). Thus, the MOS-type sensing device shows the benefit of low-temperature operation. Kinetic analyses confirm that the short response times of the MOS diode are attributed to high reaction rate at the Pd/SiO2 interface. Keywords: Pd, AlGaN, SiO2, Schottky diode, Hydrogen

The behaviors of fluorine in BF~ implanted polycrystalline silicon (poly-Si) on silicon have been investigated in the annealing temperature range of 850 to 1100~ The distribution of fluorine atoms as functions of temperature and time have been monitored by the secondary ion mass spectroscopy (SIMS) and cross-sectional transmission electron microscopy (XTEM). The XTEM micregraphs revealed that fluorine bubbles are distributed in the poly-Si and at the original poly-Si/ Si interface after annealing. The locations of bubbles were found to correspond to the fluorine peaks in the SIMS depthconcentration profiles. The presence of the boron peak at the original poly-Si/Si interface is attributed to the gettering of boron atoms by the fluorine bubbles. Moreover, the boron profiles in the silicon substrates are sensitive to thermal budget due to the pileup of fluorine atoms at the pely-Si/Si interface. The pileup of fluorine at the poly-Si/Si interface leads to an enhancement of epitaxial regrewth of poly-Si films and the formation of fluorine bubbles. Consequently, higher surface dopant concentration and deeper junction depth were obtained.

An interesting InP/InGaAs double heterojunction bipolar transistor (DHBT) with a step-graded InAlGaAs/InP collector structure are demonstrated and studied. By use of the step-graded InAlGaAs/InP collector structure at the base-collector heterojunction, the current blocking effect is effectively eliminated. Experimentally, the studied device shows a relatively better common-emitter breakdown voltage and low output conductance though at high temperature. Also, the studied device shows a wider collector current density operation region which over 11 decades in magnitude of collector current density (10~6 to 105 A/cm2). Moreover, the studied device also shows the relatively weaker temperature dependence on the electron impact ionization alpha. Consequently, the studied DHBT device provides the promise for low-voltage and low-power circuit applications.

A comprehensive study of the InGaP ledge width effect on the performance of InGaP/GaAs heterojunction bipolar transistors (HBTs) is demonstrated. It is shown that the electron density and recombination rate at the surface channel are decreased with increasing the InGaP ledge width. However, the longer InGaP ledge width increases the base-collector junction area which in turn deteriorates the high frequency performance. According to the theoretical analysis and experimental results, the device with an InGaP ledge width of 0.8 mum shows the best and acceptable DC and RF performance.

The hydrogen sensing and response characteristics of and Pt/GaN Schottky diodes with thin SiO2-passivated metal/semiconductor junction under different-concentration hydrogen gases are studied over a wide temperature range in an air atmosphere. Experimentally, the studied device exhibits hydrogen sensing performance, including SF of 14685 and SR of 44636 (in 9970 ppm H2/air), DeltaphiB of 231.6 meV (in 9970 ppm H2/air), temperature range (300 - 850 K) and voltage-operating sweep under forward and reverse bias conditions. According to the steady-state analysis on adsorption equilibrium, the studied device further shows the excellent performance for high temperature detection and improved activity of hydrogen adsorption reaction.

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 wide operating temperature range. In particular, as compared with the device only with sulfur passivation, the slightly 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. 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 Pd/oxide/AlGaAs metal oxide semiconductor (MOS) junction resistor-type hydrogen sensor, based on the current-voltage characteristics in the linear region of an AlGaAs-based PHEMT structure, is fabricated and studied. A simple model is employed to elucidate the hydrogen detection mechanism. The dissociation of H2, diffusion of H atoms and formation of a dipole layer cause a in channel resistance, show that the VGS bias the resistance sensitivity, current variation, transient response, and response time. At 30degC, a significant resistance response of 33.3% (82.8%) to 4.3 (9970) ppm H2/air is obtained at VGS=-0.6 V. Nevertheless, the largest conductance variation (DeltaG) appears to be in the range between VGS=-0.3 and -0.4 V. The transient response at VGS=-0.3 V shows larger current variations, accompanying the longer response time than those at VGS=0 V.

The temperature-dependent DC characteristics and noise performance of an interesting InGaP/GaAs heterojunction bipolar transistor (HBT) with emitter-edge-thinning structure are demonstrated. Experimentally, due to the emitter-edge-thinning structure, higher current gains and wider collector current operation regime over the measured temperature range (300 ~ 400 K) are observed as compared to the conventional device. In addition, the studied device exhibits lower base current ideality factors, better thermal stabilities on DC current gains, lower base surface recombination current densities, and improved device reliability. Therefore, the studied device is suitable for low-power and high-temperature electronic applications.

Hydrogen sensing properties of a metamorphic high electron mobility transistor (MHEMT) are studied and presented. This MHEMT-based sensor exhibits good pinch-off characteristics upon exposing to hydrogen gases. Besides, the current variation and threshold voltage shift of the studied device reveal larger response under hydrogen-containing conditions. The studied device shows fast responses and exhibits a large current variation magnitude of the order of milliamperes and a relatively low sensitivity due to the high baseline current. Based on the Langmiur isotherm, experimental current responses are consistent with the simulated curve. This indicates that the surface reaction is the rate limited factor for this hydrogen adsorption reaction.

The reliability-related properties of an InAlAs/InGaAs metamorphic high electron mobility transistor (MHEMT), using the non-annealed ohmic-recess (NAOR) approach, are studied and demonstrated. The NAOR device shows significantly improved dc and radio frequency (RF) performance over a wide temperature range (300–500 K). With a 1 × 100 µm2 gate-dimension MHEMT by the NAOR approach, the considerably improved thermal stability and dc performances, including lower temperature variation coefficients on turn-on voltage (−1.38 mV K−1) and gate-drain breakdown voltage (−30.4 mV K−1), and on-resistance (2.41 × 10−3 Ω mm K−1), are obtained as the temperature is increased from 300 to 500 K. For RF characteristics, the NAOR device also shows a low degradation rate on drain saturation current operating regimes (−5.52 × 10−4 K−1) as the temperature is increased from 300 to 400 K. In addition, based on the lifetime tests, an activation energy of 1.33 eV and a projected median lifetime of 1.2 × 107 h at Tch = 125 °C are obtained for the NAOR MHEMT.

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.

In this paper, the interesting hydrogen sensing properties of a Pd-gate AlGaN/GaN Schottky diode are investigated. A significantly low detection limit of 850 ppb H2/air gas can be observed with increasing the temperature to 423 K. The experimental results indicate that hydrogen molecules cause great influences on the diode breakdown voltage. Also, the diode exhibits an ultrahigh sensing response of 2.04×105 at 423 K when exposure to a 9660 ppm H2/air gas. The transient response time and reversibility of the studied device can be improved by increasing the operating temperature.

An interesting In0.5Al0.5As/In0.5Ga0.5As metamorphic high electron mobility transistor with non-annealed ohmic-recess (NAOR) technique is fabricated and studied. Experimentally, the parasitic resistance is substantially reduced. The used NAOR technique shows benefits of the absence of thermal treatment (non-annealed) for ohmic contact. Compared with the traditional structure, both the DC and the RF performance is significantly improved. From experimental results, the studied 1 µm gate length with NAOR technique exhibits breakdown voltage of 22.93 (16.89) V, drain saturation current of 327 (299) mA/mm and parasitic resistance of 1.67 (2.02) Ω · mm at 300 (500) K. The unity current gain cutoff frequency fT and maximum oscillation frequency fmax are 20.5 and 62.3 GHz at 300 K.

On the basis of a GaAs-based heterostructure field-effect transistor with a catalytic Pd gate film, an interesting hydrogen sensor was fabricated and studied. A study of the hydrogen-induced dipole effect on device performance, including output resistance (r 0 ), Early voltage (V A ), and the drain saturation current (I DS ) operating regime, is reported. A significant change in the relative sensitivity ratio [S r (%)] was observed in the cutoff region. In addition, a linear dependence between the logarithmic values of response time and hydrogen concentration is consistent with theoretical analysis. Experimentally, a high S r (%) of 348% in 9970 ppm H 2 /air was obtained in the cutoff region at 50°C. The width of the I DS operating regime decreased from 115.3 to 108.2 mA/mm with an increase of the hydrogen concentration from air to 9970 ppm H 2 /air. From the experimental results, it is speculated that the polarization of a dipole layer reduces the depletion region, which results in a substantial change in the two-dimensional electron gas (2DEG) and the effective channel length and shape.

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.

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.

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.