Patent ID: 7091528

Claim:
A semiconductor device that is a heterojunction bipolar transistor, said semiconductor device comprising: a collector layer; a subcollector layer; and an intermediate collector layer formed between said collector layer and said subcollector layer, wherein said collector layer and said subcollector layer are made of GaAs, wherein said intermediate collector layer is made of a semiconductor material whose band gap is larger than a band gap of GaAs, the semiconductor material of said intermediate collector layer being different than a semiconductor material of said collector layer, wherein a concentration of impurities in said intermediate collector layer is higher than a concentration of impurities in said collector layer and is lower than a concentration of impurities in said subcollector layer, and wherein the concentration of impurities in said intermediate collector layer is higher in a direction from a surface of said intermediate collector layer facing to said collector layer toward a surface of said intermediate layer facing to said subcollector layer.