US Patent 7470956 Semiconductor device and manufacturing method thereof

A semiconductor device has a semiconductor base, an anode electrode, and a cathode electrode. The semiconductor base includes a P type semiconductor substrate, an insulating film, an N− type semiconductor region formed on the insulating film, an N+ type semiconductor region, and a P+ type semiconductor region facing the N+ type semiconductor region via the N− type semiconductor region. The semiconductor device further has an N type diffusion layer which is formed, in the N− type semiconductor region at the interface between the insulating film and the N− type semiconductor region, so as to have a concentration gradient such that the N type impurity concentration increases from the side of the anode electrode to the side of the cathode electrode.