Abstract: A heterojunction bipolar transistor (HBT) with improved current gain, in which the HBT comprises a substrate, a modulation-doped buffer structure, an n-type sub-collector layer, an n-type collector layer, a p-type base layer, an n-type emitter layer, an emitter cap layer, and an emitter contact layer, a collector electrode, a base electrode and an emitter electrode, wherein the modulation-doped buffer structure includes at least one doped layer having a thickness of at least 10 ? and less than 3000 ? and doped a dopant element with doping concentration at least 3×1017cm?3 and no greater than 2×1020 cm?3, wherein the dopant element is selected from the group consisting of C, Zn, Mg, Be and S.

Abstract: A heterojunction bipolar transistor (HBT) with improved current gain and the fabrication method thereof, in which the HBT comprises a substrate, a p-type buffer layer, a sub-collector layer, a collector layer, a base layer, an emitter layer, an emitter cap layer, and an emitter contact layer. Multiple etching processes are used for etching a base electrode contact region and terminated at the base layer. A collector electrode contact region is then formed in the base electrode contact region by an etching process terminated at the sub-collector layer. A base electrode is disposed on the base layer in the base electrode contact region. A collector electrode is disposed on the sub-collector layer in the collector electrode contact region. An emitter electrode is disposed on the emitter layer.