PURPOSE: To facilitate the decrease in a threshold current of a laser transistor, the improvement in the temperature characteristic of optical output characteristic, the increase in the output and a high speed response by forming an insulating layer between a craft base and a collector.

CONSTITUTION: A first conductivity type first semiconductor layer 1, a second semiconductor layer 2 having a wider forbidden band width than the layer 1 contacted with one main surface of the layer 1, a second conductivity type third semiconductor layer 3 contacted with another main surface of the layer 1 oppositely to the layer 2, a fourth semiconductor layer 4 contacted with the side of the layer 1 with first conductivity type, and an insulating layer 13 formed on a partial portion or the entire region contacted with the layer 3 and the layer 4 are formed. For example, an N-type InP collector layer 3, a P-type InGaAsP base layer 1, and an N-type InP emitter layer 2 are sequentially formed on an N-type InP substrate 5. Then. after deeply reverse mesa etching, a semi-insulating I-type InP insulating layer 13, a P-type InP craft base layer 4 and a P-type InGaAsP cap layer 6 are formed.