Thyristor fired by overvoltage

Claim 1

A thyristor adapted to fire in response to a predetermined forward overvoltage, comprising: a body of semiconductor material having first and second outer surfaces with four impurity regions of alter-nate type conductivity disposed in PN junction relationship said regions being a cathode emitter region and a cathode base region, each having surfaces forming a portion of the first outer surface of the body, an anode base region and an anode emitter region, said anode emitter region having a surface forming at least a portion of the second outer surface of the body; said cathode emitter region including an inner auxiliary and an outer main portion, said auxiliary portion being disposed to form the central portion of the first outer surface and having an outer boundary of predetermined dimension said main portion being radially spaced from the inner portion with at least a portion of the cathode base region forming the first outer surface of the body between the auxiliary and main cathode emitter regions; an extra impurity region of the same conductivity type as the cathode base region disposed in the cathode base region centrally of the outer boundary of the auxiliary cathode emitter region portion; said extra impurity region having a higher impu-rity density gradient adjacent the PN junction formed by the cathode and anode base regions than the density gradient of the PN junction radially spaced from said extra region; a metallic electrode in ohmic contact with the first outer surface disposed overlying the PN junction formed at the outer boundary of the auxiliary cathode emit-ter region and the cathode base region; a metallic cathode electrode in ohmic contact with the first outer surface formed by the main cathode emitter region portion; and a metallic anode electrode in ohmic contact with the second outer surface formed by the anode emitter region, whereby a predetermined overvoltage applied to the anode electrode causes the device to avalanche initially in the area of the said extra region of the cathode base region.