Patent ID: 7449729

Claim:
A heterojunction bipolar transistor, comprising: a high-concentration n-type first sub-collector layer; a high-concentration n-type second sub-collector layer formed on the first sub-collector layer and made of a material having a bandgap smaller than that of the first sub-collector layer; an i-type or low-concentration n-type collector layer formed on a given portion of the second sub-collector layer; a high-concentration p-type base layer formed on the collector layer; an n-type emitter layer formed on the base layer and made of a material having a bandgap larger than that of the base layer; a high-concentration n-type emitter cap layer formed on a given portion of the emitter layer; a high-concentration n-type emitter contact layer formed on the emitter cap layer and made of a material having a bandgap smaller than that of the emitter cap layer; a first interconnection extended from the emitter contact layer and including a portion serving as an emitter electrode; a second interconnection extended from a portion of the emitter layer on which the emitter cap layer is not formed and including a portion serving as a base electrode; and a third interconnection extended from a portion of the second sub-collector layer on which the collector layer is not formed and including a portion serving as a collector electrode.