OVERVOLTAGE SELF-PROTECTION TYPE THYRISTOR

PURPOSE:To effectively turn ON a thyristor without thermal breakdown by bringing a high impurity concentration surface layer formed on a recessed surface for specifying an initially yielding zone in contact with a semiconductor layer which performs a function as an emitter adjacent to the semiconductor layer formed with a recess at the time of applying an overvoltage. CONSTITUTION:A substantially circular recess 16 is formed at a position in which N-type emitter layers 14, 15 of a P-type base layer 13 are not formed. Then, a P type surface layer 17 having a impurity concentration higher than that of the layer 13 is formed adjacently to the surface of the recess 16. The auxiliary N-type emitter layer 15 is disposed around the recess 16 to surround it, and so formed as to partly contact the layer 17. Thus, when an overvoltage is applied, the same amplifying gate mechanism as a normal turning ON is operated except a point that an avalanche current generated in the layer 17 is used as a trigger signal. Accordingly, it can be effectively turned ON even at the time of the overvoltage to protect a thyristor against a thermal breakdown.