A bistable semiconductor device comprising three or more junctions which is normally a nonconductor until the application of a signal to a gate terminal, at which time the device switches to the conductive state. Includes devices capable of being switched back to the nonconductive state upon application of a different signal to the same or another gate terminal. May or may not include mounting hardware and/or heatsink. For solid state devices which are responsive to visible or infrared radiant energy, see semiconductor device, photo.