Materials
The principal difference between the BJT and HBT is in the use of differing semiconductor materials for the emitter-base junction and the base-collector junction, creating a heterojunction. The effect is to limit the injection of holes from the base into the emitter region, since the potential barrier in the valence band is higher than in the conduction band. Unlike BJT technology, this allows a high doping density to be used in the base, reducing the base resistance while maintaining gain. The efficiency of the heterojunction is measured by the Kroemer factor, named after Herbert Kroemer who was awarded a Nobel Prize for his work in this field in 2000 at the University of California, Santa Barbara.
Materials used for the substrate include silicon, gallium arsenide, and indium phosphide, while silicon / silicon-germanium alloys, aluminium gallium arsenide / gallium arsenide, and indium phosphide / indium gallium arsenide are used for the epitaxial layers. Wide-bandgap semiconductors are especially promising, e.g. gallium nitride and indium gallium nitride.
In SiGe graded heterostructure transistors, the amount of germanium in the base is graded, making the bandgap narrower at the collector than at the emitter. That tapering of the bandgap leads to a field-assisted transport in the base, which speeds transport through the base and increases frequency response.
Read more about this topic: Heterojunction Bipolar Transistor
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