Rare-earth oxide films for gate dielectric on n-GaAs have been investigated. The oxide films were e-beam evaporated on S-passivated GaAs, considering interfacial chemical bonding state and energy band structure. Rare-earth oxides such as Gd2O3, (GdxLa1-x)(2)O-3, and Gd-silicate were employed due to high resistivity and no chemical reaction with GaAs. Structural and bonding properties were characterized by X-ray photoemission, absorption, and diffraction. The electrical characteristics of metal-oxide-semiconductor (MOS) diodes were correlated with material properties and energy band structures to guarantee the feasibility for MOS field effect transistor (FET) application. Gd2O3 films were grown epitaxially on S-passivated GaAs (0 0 1) at 400 degrees C. The passivation induced a lowering of crystallization temperature with an epitaxial relationship of Gd2O3 (4 4 0) and GaAs (0 0 1). A better lattice matching relation between Gd2O3 and GaAs substrate was accomplished by the substitution of Gd with La, which has larger ionic radius. The in-plane relationship of (GdxLa1-x) with GaAs (0 0 1) was found and the epitaxial films showed an improved crystalline quality. Amorphous Gd-silicate film was synthesized by the incorporation Of SiO2 into Gd2O3. These amorphous Gd-silicate films excluded defect traps or current flow path due to grain boundaries and showed a relatively larger energy band gap dependent on the contents of SiO2, Energy band parameters such as Delta E-C, Delta E-V, and E-g were effectively controlled by the film composition. (c) 2006 Elsevier B.V. All rights reserved.