electrodes while the remainder of the silicon nitride film is removed fig4 b the etching of the silicon nitride film 4 may be carried out after impurities are implanted into the non doped polysilicon film 1 which will be described later in this case ion implantation through the silicon nitride film 4 allows a controlled impurity implantation depth thereafter a photoresist film not shown is formed on the silicon substrate 10 and typical photolithography is used to remove the photoresist film from the n type gate region 6 a to form a resist mask 7 b with an aperture over the n type gate region then p phosphorous ions are implanted at an oblique angle of 15 degrees with respect to the direction perpendicular to the plane of the substrate fig4 c at an acceleration voltage of 10 kev and a dose of 6 0 u00d7 10 15 ions cm 2 although the ion implantation may be carried out at a normal angle to the substrate the oblique ion implantation allows the impurity to be implanted into the portion of the polysilicon film that underlies the patterned stacked film 3 and buffer layer 2 so that the surface of that portion is not unexposed consequently a uniform impurity distribution can be more easily obtained upon subsequent thermal treatment the angle with respect to the substrate may be selected as required depending on the width in the channel length.