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 µm 2 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.

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The performances of InGaP/Al x Ga 1−x As/GaAs heterojunction bipolar transistors (HBTs) with different doping concentrations of Al x Ga 1−x As graded layers are theoretically studied. The use of the Al x Ga 1−x As graded layer plays a key role in affecting the direct current and radio frequency performances of the studied HBTs. It is found that the studied devices with suitable doping concentrations of Al x Ga 1−x As graded layers exhibit lower offset voltages, saturation voltages, and base and collector current ideality factors. Furthermore, due to the use of proper doping concentrations of Al x Ga 1−x As graded layers, the studied devices show high values of the unity current gain cut-off frequency (f T ) and maximum oscillation frequency ( f max ). It is known that, from the theoretical analysis, the appropriate doping concentration of the Al x Ga 1−x As graded layer is 1 × 10 16 to 1 × 10 18 cm −3 . Consequently, this work is promising for device engineers to design high-performance HBT structures.