DC and RF characteristics of In0.5Ga0.5P/In0.22Ga0.78As pseudomorphic high electron mobility transistors (pHEMTs) having a gate oxide layer were investigated. 1.5 x 50 mum(2) gate p-HEMTs having the gate oxide thickness of 0, 50, and 300 Angstrom were fabricated by using a liquid phase oxidation technique of GaAs. Substantial improvements in gate leakage current and on-state and off-state breakdown voltage characteristics of p-HEMTs having a gate, oxide layer were observed. The on-state breakdown voltage (similar to13.2 V) of the p-HEMTs having a 50 Angstrom gate oxide layer was approximate to 2.3 times lager than that of the p-HEMTs without a gate oxide layer. While the p-HEMTs having a gate oxide layer of 300 Angstrom showed much improved gate leakage current and on-state breakdown voltage characteristics, they suffered from degradation of output conductance due to the drain induced barrier lowering originating from the thick gate oxide layer. While optimization of p-HEMT epitaxial layer structure for metal-oxide-semiconductor gate operation is required for further improvements in device characteristics, the preliminary results indicate the potential of In0.5Ga0.5P/In0.22Ga0.78As/GaAs p-HEMT having a gate oxide layer for high power applications. (C) 2002 Elsevier Science Ltd. All rights reserved.