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A new hydrogen sensor based on a GaAs-based high electron mobility transistor (HEMT) with a catalytic Pt-oxide-Al0.24Ga0.76As (MOS) gate structure is fabricated and demonstrated. The threshold voltage shift, hydrogen detection sensitivity, and transient responses of the device under different hydrogen concentrations and temperature are measured and studied. Based on the transistor amplification action, even at an extremely low hydrogen concentration of 14ppm H2/air, the studied device shows significant drain current variation (about 0.12mA). Furthermore, the studied device can be operated under wider operating temperature regimes with remarkable hydrogen-sensing properties. The decreased hydrogen detection capability with increasing operating temperature demonstrates the exothermic reaction of the hydrogen adsorption and desorption processes.

A heterostructure field-effect transistor with an n+-GaAs/p+-InGaP/n-GaAs high-barrier gate and n-GaAs/i-InGaAs/i-GaAs/delta-doped sheet heterostructure channel (device A) has been successfully fabricated and studied. The heavily doped p+-InGaP layer and heterostructure channel are introduced to increase the barrier height and carrier confinement, respectively. Experimentally, the device without the high-barrier gate structure (device B) is also fabricated for comparison to investigate the influence of high-barrier gate structure on device characteristics. Due to the high-barrier gate added to suppress the tunneling current in device A, the gate leakage current is also substantially reduced. Therefore, the turn-on voltage, breakdown voltage, current drivability, and transconductance linearity are all significantly improved for device A relative to device B. Furthermore, the temperature-dependent characteristics of device A are also improved.

The influences and improved methods for mesa-sidewall effects of GaInP/InGaAs/GaInP pseudomorphic high electron mobility transistors are investigated and demonstrated. The mesa-sidewall effects of gate leakage current path and parasitic capacitance seriously degrade the device characteristics. For instance, the excessive gate leakage current, reduced breakdown voltage and transconductance, variation of threshold voltage, increased sidegating effect, and degraded radio frequency response are found when the number of the mesa sidewall is increased. In the work, a simple and low-cost technique of the selective removal of mesa-sidewall materials is introduced. This selective etching method can substantially eliminate the mesa-sidewall contact problems of heterostructure field-effect transistor devices.