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In this paper, electrical and interfacial properties of MOS capacitors with atomic layer deposited (ALD) , , and HfAlO gate dielectrics on sulfur-passivated (S-passivated) GaAs substrates were investigated. HfAlO on p-type GaAs has shown superior electrical properties over or on GaAs, and it is attributed to the reduction of the Ga–O formation at the interfacial layer. HfAlO on p-type GaAs exhibits the best electrical properties after postdeposition annealing (PDA) at 500 . It is found that PDA, at above 500 , causes a significant amount of Ga and As out-diffusion into the high- dielectric, which degrades the interface, as well as bulk high- properties.