[en] In a substrate of semiconductor material of one conductivity type and high resistivity, a thin layer of the same conductivity and low resistivity is provided adjacent a major surface of the substrate. A region of opposite conductivity type is provided in the substrate adjacent the major surface to form a PN junction therewith spaced adjacent to the thin layer. Zero bias is provided on the PN junction. Minority charge carriers generated in the semiconductor substrate underlying the thin layer in response to applied radiation diffuse to the region of opposite conductivity type and are sensed