A gate insulator film with a wide bandgap and a high dielectric constants required to achieve high power field effect transistor (FET) using wide bandgap semiconductors such as SiC and diamond. We can achieve to suppress the gate leakage current and the charge shifts by using the AlSiO film. We found that the leakage current of Nitrogen-doped AlSiO film can be suppressed in high temperature region compared with AlSiO film. In addition, we attempted to study Yttrium aluminate (YAlO) and Lanthanum aluminate (LaAlO) as a gate insulator film. Since the Y (+3) and La(+3) have the same valence as Al (+3), it is expected that there is an advantage that Y, La doping generates less dangling bonds in the Al2O3 film. We can achieve to form aluminum based oxide film with higher dielectric constant by doping Lanthanoid atoms such as Y and La. The optimized YAlO and LaAlO films were applied to SiC-MIS structure. The Lanthanoid and Aluminum based oxide films can readily be applicable to wide bandgap semiconductor devices.