Patent ID: 6881988

Claim:
A heterojunction bipolar transistor comprising: a sub-collector region made of semiconductor of a first conductivity type; a collector region made of semiconductor of the first conductivity type in contact with the sub-collector region; a collector electrode spaced apart from the collector region and in contact with the sub-collector region; a base region made of semiconductor of a second conductivity type in contact with the collector region; an emitter region made of semiconductor of the first conductivity type in contact with the base region; the collector region including: a first collector layer made of InGaP, which contains a natural superlattice having regularly arranged In atoms and Ga atoms in a group-III atomic layer, in contact with the sub-collector region; a second collector layer made of semiconductor of the first conductivity type or undoped semiconductor, said second collector layer (a) having a larger thickness than the first collector layer, and (b) having a narrower band gap than the first collector layer, in contact with the base region; and a third collector layer made of semiconductor of the first conductivity type having a higher doping concentration than the second collector layer located between the first collector layer and the second collector layer, wherein the collector region is sandwiched between the sub-collector region and the base region.