Semiconductor device with a high breakdown voltage characteristic

A semiconductor device comprising a semiconductor substrate having two major surfaces comprising a first region of one impurity type and a second region of the opposite conductivity type, said first and second regions forming a PN junction therebetween which junction extends to one major surface of said substrate, said PN junction being arranged to be reversely biased by an applied voltage, said substrate including at least one field limiting ring of opposite conductivity type semiconductor material formed therein adjacent said one major surface and spaced from said PN junction by a distance such that the depletion layer of said PN junction is spread to said ring by said voltage, which voltage is less than the breakdown voltage, and a polycrystalline silicon layer covering an entire surface end of said PN junction, the said field limiting ring and an area of said one major surface where said depletion layer spreads, said polycrystalline silicon layer having a resistivity greater than 10.sup.4 ohms-centimeters, a layer of silicon dioxide overlying said polycrystalline silicon layer, and in which said silicon dioxide layer has a gradually changing oxygen concentration between its top and bottom.