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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

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.

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.

DC characteristics of interesting In0.49Ga0.51P-channel heterostructure field-effect transistors (HFETs) with different gate metals, e.g., Pt, Pd and Au, have been studied and demonstrated. A very thin and fresh oxide layer is introduced to increase the gate turn-on and breakdown voltages and reduce the gate leakage current. It is found that, metal-oxide-semiconductor field-effect transistors show higher barrier height, turn-on voltage, breakdown voltage, off-state voltage, output current and transconductance than metal-semiconductor field-effect transistors. In addition, the barrier height and turn-on voltage are increased with the increasing metal work function. However, the breakdown voltage is decreased with the increasing metal work function. All studied devices show good performances of high breakdown voltage, high output drain saturation and flat and wide transconductance operation regimes. The temperature-dependent behaviours are also studied. It is known from experimental results that In0.49Ga0.51P HFETs show good potentiality in high-temperature electronics applications.

A borderless contact (BLC) technique and double-implant structure (DIS) have been developed successfully to fabricate high-performance Ti-salicide sub-quarter-micron CMOS devices. A SiOxNy film grown by low-temperature chemical vapour deposition is used to act as the selective etch-stop layer. The n+ and p+ DIS can reduce the junction leakage current which is usually enhanced by BLC etching near the edge of shallow trench isolation. Based on the use of the BLC process, the process window can be enlarged. In addition, the employed low-thermal oxynitride and high deposition rate can improve the salicide thermal stability and avoid the salicide agglomeration. Experimentally, by combining the BLC and DIS techniques, low leakage and low sheet resistance CMOS devices and low standby current and high yield 1 Mb SRAMs are fabricated successfully.

A novel and high-performance Pd/oxide/GaAs hydrogen sensor based on a metal-insulator-semiconductor field-effect transistor (MISFET) is fabricated and studied. In the presence of the interfacial oxide, high sensitivity and significant increase in output drain current are observed. In the presence of hydrogen, a 2×200 µm2 gate dimension device shows good dc characteristics including high turn-on voltage, an obvious variation of drain current and a short response time. In addition, under the applied voltage of -4 V and 537 ppm hydrogen in air, a very high sensitivity of 9473 is obtained. This performance shows that the device studied has a good potential for high-speed and high-sensitivity hydrogen sensor and MISFET integrated circuit applications.

A novel narrow-base heterojunction bipolar transistor and a multiple-state switching device based on the InP/InGaAlAs material system have been fabricated and demonstrated. Common-emitter current gains up to 28 and 25 are obtained. Due to the use of an InGaAlAs narrow base, interesting topee-shaped current-voltage characteristics presented in the transistor active regime and negative-differential-resistance (NDR) loci are observed under the applied base current of IB = 2 µA/step. In addition, multiple-route and multiple-state S-shaped NDR characteristics are observed at 77 K due to the insertion of the InGaAs quantum well in the base-collector junction. The topee-shaped NDR phenomena are attributed to the modulation of the potential spike at the base-emitter heterojunction. However, the switching behaviours are due to the avalanche multiplication, confinement effect and two-stage barrier lowering effect.

Temperature-dependent dc performances of lattice-matched InP/InGaAlAs heterojunction bipolar transistors (HBTs) using the InGaAlAs quaternary alloy as the base and collector layers are studied and reported. When compared with conventional InP/InGaAs HBTs, the device studied exhibits a higher common-emitter breakdown voltage and a lower output conductance even at high temperature. The variations of offset voltage and ideality factor at different temperatures have been analysed. In addition, with decreasing temperature from 25 °C toward -196 °C, an irregular temperature behaviour of current gain is observed. At high current levels, the temperature-dependent current gain is mainly determined by the reduced reverse hole injection current. As the current level is lowered, the dominance of reverse hole injection current is correspondingly replaced by the recombination current.

The temperature-dependent characteristics of an InP/InGaAs superlattice-emitter resonant-tunnelling bipolar transistor have been studied and demonstrated. Due to the use of a five-period InP/InGaAs superlattice, the RT effect is observed at cryogenic temperature. In addition, the temperature-dependent dc characteristics of the studied device from room temperature to 398 K are reported. Dc current gain remains at an approximately constant value over the measured temperature range. The temperature coefficients of base-emitter and base-collector turn-on voltages are -2 and -3 mV K-1, respectively. The ideality factors of base current (nB) and collector current (nC) exhibit negative temperature coefficients. nB≈1 indicates that the bulk base recombination current component dominates the whole base current as the temperature is increased.

A newly designed high-barrier-gate Ga0.51In0.49P/InxGa1-xAs/GaAs pseudomorphic transistor with a step-compositioned channel (SC2) and bottomside delta-doped sheet (BD2S) structure has been fabricated successfully and studied. For a 1×100 µm2 studied device, a high gate-to-drain breakdown voltage over 30 V is found. In addition, an available output current density up to 826 mA mm-1 at a high gate voltage of 2.5 V, a maximum transconductance of 201 mS mm-1 with a very broad transconductance operation regime of 3 V of gate bias (565 mA mm-1 of drain current density) and a high dc gain ratio of 575 are obtained, simultaneously. Meanwhile, the maximum values of the unity current-gain cut-off frequency fT and oscillation frequency fmax are 16 and 34 GHz, respectively.

Novel double delta-doped sheet (D3 S) Ga0.51 In0.49 P/In0.15 Ga0.85 As/Ga0.51 In0.49 P pseudomorphic high-electron-mobility transistors (PHEMTs) have been fabricated successfully and studied. A wide-gap Ga0.51 In0.49 P Schottky layer and a D3 S structure are used to improve device performance. Furthermore, an airbridge-gate structure is employed to achieve good dc and RF performances. For a 1 µm gate length device, a high gate-to-drain breakdown voltage over 35 V, an available output current density up to 615 mA mm-1 , a maximum transconductance of 110 mS mm-1 and a high dc gain ratio of 487 are obtained. On the other hand, the maximum values of unity current gain cut-off frequency fT and maximum oscillation frequency fmax are 19.5 and 40.5 GHz, respectively. The output power of 15.6 dB m (363 mW mm-1 ), power gain of 5.6 dB, power added efficiency (PAE) of 37% and drain efficiency (DE) of 51% are obtained at an input power of 10 dB m and the measured frequency of 2.4 GHz.