PURPOSE: To obtain high speed operation, by changing continuously the band gap and the refraction index of the base layer and forming its central part with the semiconductor thin film layer as the quantum well layer.

CONSTITUTION: The P-type InGaAs quantum well layer 101 is sandwiched between the P-type GaAsP distributed refraction index layers 102, and the base layer is constituted by the layers 101 and 102. The N-type InP emitter layer 103 and the N-type collector layer 104 are formed in contact with the layer 102. From the P-type InP graft base layer 106, the current is supplied to the layer 102. The band gap energy of the layer 101 is small as compared with that of its both side layers 102, so that it captures the carrier easily. As regards the distribution of the refraction index, the layer 101 has a protruded configuration as composed with other layers and the effective confinement of light is made in this layer. Accordingly, the sufficient carrier can exist in the base without making the laser transistor Tr or the light emitting transistor in the saturation state, and the high speed operation is made possible.