The optical semiconductor device comprises cathode (23) and anode regions (26), and a gate region (25) having a forbidden band gap narrower than those of the cathode and anode regions. The gate region (25) is sandwiched between the cathode and anode regions (23, 26). The optical semiconductor device further comprises a plurality of windows (20A, 20B) for receiving and radiating light so that the alignment of optical axes can be easily performed for a plurality of input (1A) and output (1B) light beams. Various optical logic, memory and other devices are disclosed.