The band gap and band offset of Gd2O3 gate oxide film on n-GaAs (001) could be controlled by the incorporation of SiO2. Prior to the deposition of the gate oxide, the GaAs surface was passivated with sulfur. From the observation of 3p photoelectron lines of Ga and As, S-passivated GaAs surface was found to be not-oxidized during the formation of the gate oxide films. The photoelectron binding energy shift of Gd 4d was observed when SiO2 was incorporated into Gd2O3 and this could be explained by the different electronegativity of second nearest neighbor element, in this work, Si. Energy band gaps of Gd2O3 and (Gd2O3)(0.5)(SiO2)(0.5) were measured as similar to 5.4 and similar to 6.8 eV, respectively, using energy loss spectra of O 1s photoelectron lines. The change in the energy band structure of Gd2O3 with regard to n-GaAs due to the incorporation of SiO2 was demonstrated by correlating band gap values and valence-band maximum energy. An enhanced conduction-band offset of (Gd2O3)(0.5)(SiO2)(0.5) resulted in the decrease of leakage current density. Fowler-Nordheim tunneling mechanism was applied to explain the increase of barrier height. (c) 2005 Elsevier B.V. All rights reserved.