Disclosed is a Gunn-effect device which is sensitive to light, comprising a semiconductor such as a crystal of N-type gallium arsenide having high-resistance, alloyed contacts of, for example, indium and gold, characterized by the capability of being switched on and off by regulating the bias voltage across the crystal and the incident light on narrow portions of the crystal. In particular, incident light on the domain-forming and domain-terminating portions of the crystal is adjusted as to selectively foster or inhibit oscillation of the crystal.