Heterojunction bipolar transistor

A heterojunction bipolar transistor comprising:

• a first emitter region;
• a second emitter region extending on the first emitter region and connected to the first emitter region via a first junction, the second emitter region having a forbidden band gap wider than a forbidden band gap of the first emitter region, wherein, at the first junction, the second emitter region has a carrier energy level substantially equal to a carrier energy level of the first emitter region;
• a third emitter region extending on the second emitter region and connected to the second emitter region via a second junction, the third emitter region having a forbidden band gap narrower than the forbidden band gap of the second emitter region, wherein, at the second junction, the third emitter region has a carrier energy level substantially equal to a carrier energy level of the second emitter region;
• an intrinsic base region extending on the third emitter region and having a forbidden band gap narrower than the forbidden band gap of the third emitter region;
• a collector region extending on the intrinsic base region;
• an extrinsic base region extending outward of the intrinsic base region, wherein a portion of the extrinsic base region which adjoins the second emitter region has a forbidden band gap substantially equal to the forbidden band gap of the second emitter region, and wherein the extrinsic base region contacts the intrinsic base region, the second emitter region, and the third emitter region but is separated from the first emitter region;
• and
• a high-resistivity region extending underneath the extrinsic base region.