Abstract: A semiconductor controlled rectifier of an asymmetrical construction which, only when supplied with a reverse surge voltage, is turned on in the reverse direction comprising a semi-conductor body having three contiguous regions forming an inner region of one conductivity type and two outer regions of the opposite conductivity type, a first external region of the one conductivity type provided adjacent a first one of said outer regions, a second external region of the one conductivity type provided adjacent a second one of said outer regions, said external regions being positioned in such a manner that a portion of each said outer regions is exposed, a cathode electrode in conductivity contact with said first external region, an anode electrode in conductive contact with said second external region and said exposed portion of the said second outer region, and a gate electrode in conductive contact with said exposed portion of first outer region, the cathode electrode and the second external region overlapping