The selective oxidation of AlGaAs using liquid phase oxidation and photoresist or metal as a mask is investigated, and the further application of this process for the fabrication of AlGaAs/InGaAs metal-oxide-semiconductor pseudomorphic high electron mobility transistors (MOS-PHEMTs) is explored. The stability of the oxide film after exposure to the developer solution is also evaluated. The measured f(T) and f(max) are 13.9 (12) and 59 (30) GHz at maximum transconductance g(m) for MOS-PHEMT (PHEMT), respectively. In comparison, the AlGaAs/InGaAs MOS-PHEMT is a good candidate for high speed and low noise applications. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3185855] All rights reserved.