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. Wide- bandgap semiconductors such as gallium nitride and indium gallium nitride are especially promising. Materials used for the substrate include silicon, gallium arsenide, and indium phosphide, while silicon / silicon-germanium alloys, aluminum gallium arsenide / gallium arsenide, and indium phosphide / indium gallium arsenide are used for the epitaxial layers. Kroemer was awarded a Nobel Prize in 2000 for his work in this field at the University of California, Santa Barbara. The efficiency of the heterojunction is measured by the Kroemer factor. Unlike BJT technology, this allows a high doping density to be used in the base, reducing the base resistance while maintaining gain. 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. 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. Barriers indicated for electrons to move from emitter to base, and for holes to be injected backward from base to emitter Also, grading of bandgap in base assists electron transport in base region Light colors indicate depleted regions. Materials Bands in graded heterojunction npn bipolar transistor. Detailed theory of heterojunction bipolar transistor was developed by Herbert Kroemer in 1957. The idea of employing a heterojunction is as old as the conventional BJT, dating back to a patent from 1951. It is commonly used in modern ultrafast circuits, mostly radio frequency (RF) systems, and in applications requiring a high power efficiency, such as RF power amplifiers in cellular phones. The HBT improves on the BJT in that it can handle signals of very high frequencies, up to several hundred GHz.
The heterojunction bipolar transistor ( HBT) is a type of bipolar junction transistor (BJT) which uses differing semiconductor materials for the emitter and base regions, creating a heterojunction.
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