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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 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.

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

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 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.

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.

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 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.

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.