An effect arising when a junction exists between two dissimilar materials and one of the materials is exposed to electromagnetic radiation, usually in the range near-ultraviolet to infrared. The two materials may, for example, be a metal and a semiconductor or two semiconductors of opposite polarity, the combination forming a Schottky barrier and a P-N junction respectively. A forward voltage appears across the illuminated junction and power can be delivered to an external circuit.
The incident radiation imparts energy to electrons in the valance band, and electron-hole pairs are generated in the depletion region existing around the P-N junction and in the Schottky barrier. As the pairs are produced they are able to cross the junction and produce the forward bias. A migration of electrons into the n-type semiconductor produces a negative bias while an excess of holes migrating into the p-type semiconductor of the metal produces a positive bias.