Light fired thyristor with faulty firing protection

Claim 1

In a light fired thyristor including: a semiconductor body having four zones of alternatingly opposite conductivity types including a first emitter zone, a control base zone adjacent thereto, a main base zone adjacent said control base zone and a second emitter zone adjacent said main base zone, said first emitter zone being formed within said control base zone at one major surface of said semiconductor body and being divided into a plurality of laterally spaced separate regions, so that said control base zone extends to said one major surface between said regions of said emitter zone and adjacent the edges of said semiconductor body; an ohmic contact for said second emitter zone; a first ohmic contact on said one major surface and contacting only a portion of the surface of one of said regions of said first emitter zone, a further portion of said surface of said one of said regions of said first emitter zone being exposed to provide a radiation receiving surface so that said one region serves to fire the thyristor in response to impinging radiation energy; and a second ohmic cathode contact on said major surface ohmically contacting the remaining regions of said first emitter zone and the portions of said control base zone extending therebetween to said one major surface to form short circuits with said control base zone the improvement comprising: means for providing compensation for an increased potential in said control base zone below said one region of said first emitter zone caused by an interfering potential due to the dv/dt loading or the higher blocking current resulting from higher temperature, said means including a third ohmic contact on said major surface near the edge of said semiconductor body and contacting said control base zone and positioned so that a compensation potential of at least 50% of said interfering potential, and in the same direction, is produced at said third ohmic contact, and means for ohmically connecting said first and third ohmic contacts, whereby said compensation potential is applied to said one region of said first emitter zone so that its potential is raised to approximately the interfering potential of said control base one beneath said one region of said first emitter zone.