A new majority‐carrier photodetector, called a modulated barrier photodiode (MBP), grown by molecular beam epitaxy has been developed. In sharp contrast to a bipolar phototransistor, the optical gain of MBP increases with decreasing incident power. An optical gain of 1000 at 1.5 nW incident power and a fall time of 600 psec have been obtained. In addition to the application for uses as a detector in an optical communication system, the device points to the feasibility of realizing a solid‐state triode.