PURPOSE: To obtain an optical memory having high trigger optical sensitivity by providing functions of receiving a trigger light from a layer thicknesswise direction, i.e., planar direction to start a laser oscillation, and continuing the oscillation even after the light is eliminated.

CONSTITUTION: The forbidden band energies Eg=E4=E1 of a p2 layer for an anode and an n1 layer for a cathode are provided. A gate is formed by superposing a first layer of impurity concentration p1 and Eg=E2, a second layer of p11 and E21 and a third layer of n2 and E3, and forming a fourth layer of Eg=E22<E21, E23 in the second layer. In this configuration, a bias voltage is set to V=VX, suitable amount of light is incident and absorbed by the gate. Generated holes are injected to the p1 layer to increase an electron current, an avalanche amplification is generated in the third layer to increase the holes injected from the anode to the gate. An element is turned ON by a trigger light by this positive feedback to perform a laser oscillation. The nearer the VX approaches the breakover voltage VBO, the higher the trigger sensitivity becomes. Since the fourth layer of p12 and E21 is isolated from the third layer of n2 and E3, a high voltage is effectively applied at the time of OFF to prevent the VBO from rising.