Patent US 4,841,350

In a photothyristor provided with a cathode, a gate and an anode, the impurity densities of gate (base) portions 20 and 21 are made unequal to each other. The minority carriers are stored in the high impurity density regions 20, the majority carriers are permitted to readily pass through the low impurity density regions 21, and the high and low impurity density regions are electrically coupled together. This thyristor is extremely high in sensitivity and high-speed in operation.