A gate insulator film with a wide bandgap and a high dielectric constant is required to achieve high-power field effect transistors (FET) using wide bandgap semiconductors such as SiC, GaN, and diamond. It is observed that an aluminum silicon oxide (AlSiO) film containing 11% nitrogen has a high resistivity of 5 x 10(15) Omega cm, and the leakage current of a nitrogen-doped aluminum silicon oxide (AlSiON) film is also suppressed at high temperature, as compared to the AlSiO film. For example, the leakage current at 240 degrees C is four orders of magnitude smaller than that of the AlSiO film, suggesting that the AlSiON film is applicable to high temperature operation of wide bandgap semiconductor devices. (C) 2010 Elsevier B.V. All rights reserved.