Abstract: This invention relates to high breakdown voltage semiconductor devices and consists in a semiconductor device formed with a highly doped impurity region of the same conductive type as the semiconductor substrate, wherein the highly doped impurity region projects by a prescribed amount opposite a first impurity region and is formed at the back face of the semiconductor substrate, its projecting width T being in the same position as the middle of the first impurity region and being such as to satisfy t1.ltorsim.T.ltorsim.t1+2W0, where t1 is the width of the first impurity region, and W0 is the separtion in the depth direction between the first impurity region and the highly doped impurity region.