HBTs

HBTs, or heterojunction bipolar transistors, are a class of bipolar junction transistors in which the emitter and base are made from different semiconductor materials, creating a heterojunction. This structure improves emitter injection efficiency and reduces base transport losses, yielding higher current gain and faster switching, particularly at high frequencies.

Most HBTs use a wide-bandgap emitter material paired with a narrower-bandgap base, with the collector material chosen accordingly. In common III-V implementations, a wide-gap emitter such as AlGaAs or AlInGaP is connected to a GaAs or InP base. The heterojunction suppresses hole injection from base to emitter and reduces recombination, which enhances high-frequency performance. Silicon-based HBTs use a SiGe alloy in the emitter or base to form a heterojunction with silicon, enabling similar gains in speed and efficiency within silicon processes.

Typical devices are fabricated as NPN or PNP transistors, with the emitter heavily doped, the base lightly doped, and the collector moderately doped. Operation resembles that of conventional BJTs, but the heterojunction design strengthens emitter injection and can reduce base resistance and the nonlinearities associated with base transport.

Applications of HBTs span high-speed analog and digital circuits, RF and microwave amplifiers, and optoelectronic integrated circuits. III-V HBTs are common in wireless transceivers, satellite receivers, and various microwave/mm-wave modules, while SiGe HBTs are widely used in consumer electronics and integrated mixed-signal ICs.

HBT technology emerged in the 1980s as a faster alternative to conventional silicon BJTs, with continued development across material systems and integration approaches to support advancing communications and sensing technologies.