Patent ID: 7728357

Claim:
A heterojunction bipolar transistor comprising: a sub-collector layer; a collector layer formed on said sub-collector layer; a base layer formed on said collector layer; and an emitter layer, which is formed on said base layer and is made of a semiconductor that has a larger bandgap than a semiconductor of said base layer, wherein said collector layer includes: a first collector layer formed on said sub-collector layer; a second collector layer formed on said first collector layer; and a third collector layer formed between said second collector layer and said base layer, wherein said first collector layer is made of InGaP having a disordered structure and an impurity concentration equal to or higher than an impurity concentration of said sub-collector layer; a thickness of said first collector layer ranges from 5 nm to 30 nm inclusive; said second collector layer and said third collector layer are made of GaAs; a thickness of said second collector layer is 400 nm or more; a thickness of said third collector layer is 600 nm or less; and an impurity concentration of said second collector layer is lower than the impurity concentration of said sub-collector layer and higher than an impurity concentration of said third collector layer.