Semiconductor device having a PN junction formed on an insulator film

Claim 1

A semiconductor device including a vertical bipolar transistor which comprises: a semiconductor substrate of a first type of conductivity; a highly doped region having a second type of conductivity, said highly doped region being formed in said substrate; an insulation film which is formed on said substrate and which has an opening through which at least one portion of said highly doped region extends; a first single crystal semiconductor region having the second conductivity type which is formed on said insulation film and which is electrically connected to said highly doped region through said opening, wherein said first single crystal semiconductor region has an impurity region of the second type of conductivity adjacent to an upper surface of said first single crystal semiconductor region, thereby to form an emitter or a collector for said bipolar transistor, and has an impurity region of the first type of conductivity which surrounds said impurity region of the second type of conductivity thereby to form a base region for said bipolar transistor, and wherein a portion of the first single crystal semiconductor region of the second type of conductivity in contact with said highly doped region forms a collector or an emitter, said portion of the first single crystal semiconductor region having an impurity concentration which is less than that of the highly doped region; a polycrystalline silicon layer having the first conductivity type which is formed to electrically contact an outer periphery of said first single crystal semiconductor region; and a graft region which is formed in a portion of said first single crystal semiconductor region between polycrystalline silicon layer and the base and collector regions formed in the first single crystal semiconductor region, wherein said graft region is formed by diffusion from said polycrystalline silicon layer into the first single crystal semiconductor region, wherein a first contacting surface between said graft region and said polycrystalline silicon layer exists on said insulation film at a location spaced away from said opening so that said first contacting surface does not come within said opening, and further wherein a second contacting surface between said graft region and said collector region formed in said first single crystal semiconductor region adjoining said graft region exists on said insulation film at a location spaced away from said opening so that said second contacting surface does not come within said opening to thereby prevent direct physical contact between said graft region and said highly doped region and to prevent downward diffusion from the graft region towards said highly doped region.