A layer of transparent conductive material insulatingly overlies a major surface of a substrate of semiconductor material to provide a CIS (conductor-insulator-semiconductor) capacitor. A region of opposite conductivity type is provided in the substrate adjacent the major surface of the substrate. The capacitor is biased in accumulation and the region of opposite conductivity type is reversely biased with respect to the substrate. Minority charge carriers generated in the semiconductor substrate underlying the conductive layer in response to applied radiation diffuse to the region of opposite conductivity type and are sensed.