ABSTRACT
Heterojunction bipolar transistors (HBT) differ from conventional bipolar junction transistors (BJT) in their use of hetero-structures, particularly in the base-emitter junction. The energy gap difference in the base-emitter heterojunction of an HBT burdens the holes from the base to experience a much larger energy barrier than the electrons from the emitter. The exponential factor is the key to the fact that base doping can be made larger than the emitter doping without adversely affecting the HBT performance. A BJT has two key current components. The first is the forward-injection current conducted by the electrons. The second current component is composed of holes that are back-injected from the base to the emitter. The useful collector current in the homojunction transistor is only 1/6 of the undesirable back-injection current. The breakdown voltage represents the absolute maximum bias that can be applied to a bipolar transistor.
