a back electrode 13 made of metal the plane distribution of the p type semiconductor regions 31 may be other than the distribution shown in fig1 a for example a p type semiconductor region group 32 shown in fig6 a has only four p type semiconductor regions 31 facing to the corners 5a of a pair of p type semiconductor regions 5 this is because a depletion layer extending into the n type semiconductor region 3b generates the largest electric field in the areas facing to these corners 5a a p type semiconductor region group 33 composed of two rectangular ring arrays of the p type semiconductor regions 31 is shown in fig6 b the regions 31 which belong to the inner rectangular ring array and the other regions 31 which belong to the outer rectangular ring array are disposed in staggered fashion hence the extension width of the depletion layer is further increased in a mosfet 210 shown in fig7 a a semiconductor region group 34 comprising an array of p type buried semiconductor regions 32 is formed in the n type layer 3b as shown in fig7 b which is a cross sectional view taken along the line a4 a4 of fig7 a the burying depth of the regions 32 is substantially equal to the depth of the bottom of the p type semiconductor regions 5 in the case where such burying structure is adopted an advantage.