Radiation-hardened semiconductor device

A radiation-hardened semiconductor device having an N channel MOS transistor, comprising:

• a first P type region of a first boron concentration formed by implanting boron by a first high energy in the boundary of P type substrate or P type well forming said N channel MOS transistor and a field oxide film formed at the surface side of said P type substrate or P type well, and
• a second P type region of a second boron concentration lower than that of said first region formed by implanting boron by a second high energy, lower than said first high energy, near the center in the thickness direction of said field oxide film, wherein said field oxide film, said first P type region and said second P type region are all spaced from an N type diffusion layer composing the source and drain of said N channel MOS transistor, and wherein the size of said first P type region in plan area as seen from the surface side of said semiconductor device is less than the size of said second P type region in plan area, and said second P type region completely covers said first P type region and overlaps edges of said first P type region.