A GaAsp‐n‐i‐ndiode which operates as a light‐activated light‐coupled switch is described. Thei‐region is compensated semi‐insulating GaAs and is obtained by diffusing Cu, Fe, or Cr into low‐dopedn‐GaAs. The electrical and optical properties of this diode have been studied as well as its switching behavior. The diode exhibits a current‐controlled negative resistance region with ratio of off‐ to on‐resistance as high as 106. Turn‐on voltages range from 10 to 200 V, and the turn‐on time can be made as short as 10 nsec with sufficient overvoltage. The current is limited in the high‐resistance mode by intrinsic or one‐carrier space‐charge‐limited conduction in theiregion. At higher currents the light emission at thep‐njunction causes photoconduction in theiregion which is enhanced by the trapping of holes at acceptor levels. In the low‐resistance mode all the traps (approximately 1016cm−3) are filled and remain that way because of the high efficiency of light emission from the junction at the high currents. A model is developed which semiquantitatively describes both the static and dynamic properties of the diode.