We have extended our invented high k dielectrics Ga2−xGdxO3 oxide to compound semiconductors of AlGaAs, InGaAs, and InP, and demonstrated the formation of low leakage, insulating barrier on the (100) surface. Fundamental studies of the MOS diodes consisting of Au/GGG mostly on InGaAs are reported, including the crystal-chemical and electrical properties to address the oxide reliability issue. An atomically abrupt interface between the oxide/GaAs interface was achieved by the MBE growth. Systematic post annealing studies of varying temperature and gas species were carried out to improve the dielectric performance such as the reduction of frequency dispersion, voltage hysteresis, and interfacial state density Dit. We show that our results are essential to device processing based on the oxide, and to establish a viable GaAs MOSFET technology.