A semiconductor device formed on a silicon substrate consisting of the steps of producing a silicon oxide film on the silicon substrate, producing a thin silicon nitride film on the silicon oxide film, thermally nitriding the silicon nitride film in an atmosphere of nitrogenous gas, producing a conductive film on the silicon nitride film nitrided in the atmosphere of the nitrogenous gas, producing a gate region from the silicon oxide film, the silicon nitride film, and the conductive film, a channel region being positioned under the gate region in the silicon substrate, producing a source region in the silicon substrate adjacent to one side of the channel region, producing a drain region in the silicon substrate adjacent to another side of the channel region, and producing wiring regions on the source region, the drain region, and the gate region.