Semiconductor device

A semiconductor device comprising:

• a semiconductor layer;
• a plurality of semiconductor elements formed on said semiconductor layer;
• an isolation film formed in a surface of said semiconductor layer, said semiconductor elements being electrically isolated from each other by said isolation film;
• a PN junction portion formed by two semiconductor regions of different conductivity types in said semiconductor layer provided under said isolation film;
• and
• a polysilicon film provided in a position opposed to a top of said PN junction portion with said isolation film interposed therebetween across said two semiconductor regions, said polysilicon film covering a part of an upper portion of said isolation film, and a remaining part of said upper portion of said isolation film has a width smaller than twice of a thickness of said isolation film.