This study demonstrates an AlGaAs/InGaAs metal-oxide-semiconductor pseudomorphic high-electron-mobility transistor (MOS-PHEMT) with an oxidized GaAs gate by liquid phase oxidation (LPO). With the help of LPO, the threshold voltage (V-th) can be shifted positively to 0.3 V to form an enhancement-mode MOS-PHEMT. The device possesses a gate metal of 1 x 100 mu m(2) showing a drain current density of 93 mA/mm at V-GS = 2.5 V and V-DS = 7 V and a transconductance (g(m)) of 50 mS/mm at VDS = 4 V. It also exhibits an improved breakdown voltage and a suppressed gate leakage current compared to the Schottky-gate AlGaAs/InGaAs pseudomorphic high-electron-mobility transistor (PHEMT). (c) 2010 The Electrochemical Society. [DOI: 10.1149/1.3516639] All rights reserved.