Recently, the authors have investigated the GaAs metal-oxide-semiconductor field-effect transistor using Si interface passivation layer (IPL) and HfO2 as gate dielectric. In this work, they have investigated InGaAs MOSCAP using the same oxide of HfO2 as gate insulator with Si IPL. In this work, the authors studied the effects of postdeposition anneal (PDA) time and Si IPL on the electrical characteristics of the metal-oxide-semiconductor capacitor with high-k HfO2) material on InGaAs. Excellent electrical characteristics with thin equivalent oxide thickness (similar to 2.5 nm), low frequency dispersion (<5 %) have been obtained. The thickness of the Si IPL and PDA time were correlated with C-V characteristics. (C) 2007 American Vacuum Society.