Heterojunction bipolar transistor

A heterojunction bipolar transistor comprising:

• a collector layer, a base layer, an emitter layer, and a semiconductor layer that are laminated in this order, wherein
• the emitter layer includes a first region having an upper surface on which the semiconductor layer is laminated, and a second region being adjacent to the first region and having an upper surface that is exposed with respect to the semiconductor layer,
• the first and second regions of the emitter layer have higher doping concentrations in portions near the upper surfaces than in portions near an interface between the emitter layer and the base layer so as to avoid generation of a conductive region in the second region of the emitter layer and to maintain the second region of the emitter layer to be fully depleted,
• the emitter layer contains a same main component in both the portions near the upper surfaces and the portions near an interface between the emitter layer and the base layer, and
• the portions near the upper surfaces have lower doping concentrations than the semiconductor layer.