A bipolar junction transistor that incorporates a wide band gap emitter. The emitter-base junction is a heterojunction between semiconductors of different energy band gap. The following are typical material systems:
The wider band gap of the emitter reduces significantly the injection of majority carriers from base to emitter, thus maximizing the desired injection of carriers from emitter to base. This eliminates the requirement for a heavily doped emitter to achieve the same result, and consequently allows the base doping to be increased. An increase in base doping is desirable from a device viewpoint as the base resistance can be reduced significantly. This leads to an improvement in the high-frequency performance of the transistor. HBTs are used at radio- and microwave frequencies, in integrated circuit and power applications, and in optoelectronic ICs.