To improve injection efficiency, the emitter of a bipolar transistor needs to have a very high doping level. Beyond a critical doping level, the emitter energy gap decreases and, as a result, the injection efficiency drops. The heterojunction bipolar transistor (HBT) was proposed by Shockley in 1951, and was analyzed in more detail by Kroemer in 1957. Practical HBTs started to be built in the mid-1970s with the emergence of LPCVD, MBE and MOCVD technologies. They incorporate a heterojunction in the emitter-base junction, with the emitter having a larger energy gap.
The base region can be doped more heavily to reduce the base resistance. The emitter can also be doped more lightly to reduce the emitter capacitance. The injected electrons have higher energy and higher velocity to reduce the base transit time, resulting in an intrinsically faster device. An additional heterojunction at the collector, sometimes called double-heterojunction bipolar transistor (DHBT), minimizes minority carriers injected into the base from the collector in the saturation mode. HBTs are usually realized in compound semiconductors due to the availability of heterojunctions of minimal lattice mismatch. HBT on Si bipolar has been studied using SIPOS (semi-insulating polycrystalline-Si) as the emitter.
