Semiconductor device having a substrate covered with a high impurity concentration first polycrystalline layer and then a lower impurity concentration second polycrystalline layer

Claim 1

A semiconductor structure comprising: a semiconductor substrate being of a first conductivity type; a first insulation layer formed on said substrate and having at least one opening exposing a portion of said substrate; a second insulation layer formed on said first insulation layer and having at least one opening in a position corresponding to said opening in said first insulation layer, said first and second insulation layers being etchable with different etchants so that said second insulation layer is used as a mask for forming said opening in said first insulation layer by etching, a portion of said second insulation layer overhanging said first insulation layer as a result of said etching; a first polycrystalline semiconductor layer formed on said exposed substrate and containing impurities associated with a second conductivity type opposite said first conductivity type at a first concentration sufficiently high to convert the conductivity type of said substrate upon diffusion; a second polycrystalline semiconductor layer formed on said first polycrystalline layer, and containing impurities associated with said second conductivity type at a second concentration lower than said first concentration, the ratio of said first concentration and said second concentration being in the range of 1.5 to 100, said second polycrystalline semiconductor layer being three to ten times thicker than said first polycrystalline semiconductor layer; a metal electrode formed on said second polycrystalline layer; and a diffused region being of said second conductivity type formed beneath said first polycrystalline layer in said substrate, said second polycrystalline layer formed on said first polycrystalline layer prohibiting shorting between said metal electrode and said substrate and preventing breakage with respect to said overhanging portion of said second insulation layer.