A radiation detector includes a semiconductor substrate which contains front and rear major surfaces and at least one side surface, a guard ring and a plurality of anode electrode pixels located over the rear surface of the semiconductor substrate, where each anode electrode pixel is formed between adjacent pixel separation regions, a side insulating layer formed on the at least one side surface of the semiconductor substrate, a cathode electrode located over the front major surface of the semiconductor substrate, and an electrically conductive cathode extension formed over at least a portion of side insulating layer, where the cathode extension contacts an edge of the cathode electrode. Further embodiments include various methods of making such semiconductor radiation detector.