Patent ID: 6876012

Claim:
A hetero-bipolar transistor having a collector layer, a base layer and an emitter layer on a semiconductor substrate, a band gap energy of the emitter layer being greater than a band gap energy of the base layer, a carrier depleted layer being formed by a junction between the base layer and the emitter layer, comprising: an intermediate layer provided on the emitter layer, the intermediate layer having a first carrier concentration and a band gap energy smaller than the band gap energy of the emitter layer; and an emitter contact layer provided on the intermediate layer, the emitter contact layer having a second carrier concentration greater than the first carrier concentration of the intermediate layer and a thickness, wherein the first carrier concentration of the intermediate layer is determined such that the carrier depleted layer formed by the junction between the base layer and the emitter layer extends beyond the intermediate layer, and the first carrier concentration of the intermediate layer decreases monotonically from an interface between the emitter contact layer and the intermediate layer, to the emitter layer.