Abstract: There is disclosed a method for fabricating a MOSFET, comprising the steps of: forming a gate oxide on a semiconductor substrate; depositing an intrinsic semiconductor layer on said gate oxide; forming a doped semiconductor layer on said intrinsic semiconductor layer; annealing said intrinsic semiconductor layer and said doped semiconductor layer, to diffuse impurities within said doped semiconductor layer into said intrinsic semiconductor layer; and patterning said intrinsic semiconductor layer and said doped semiconductor layer, to form a gate electrode, whereby the property degradation of gate oxide attributable to impurities, the increase in the thickness of gate oxide film attributable to infiltration of impurities and the degradation in operation of device can be prevented.