Structural and electrical properties of HfO2 and HfO2/Gd2O3 gate stacks on p-GaAs substrates have been investigated. It has been demonstrated that the presence of thin layer of Gd2O3 between HfO2 and GaAs improves metal-oxide-semiconductor device characteristics such as interface state density, accumulation capacitance, frequency dispersion, and leakage current. It is also found that HfO2/Gd2O3 stack can reduce the interfacial GaAs-oxide formation, thus reduce the outdiffusion of elemental Ga and As during post-thermal annealing process. Such suppression of outdiffusion significantly improves the electrical properties of the dielectric stacks. (c) 2007 American Institute of Physics.