Heterojunction bipolar transistor
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The heterojunction bipolar transistor (HBT) is a type of bipolar junction transistor (BJT) which uses different semiconductor materials for the emitter and base regions, making a heterojunction. The HBT is better than the BJT because HBT can handle signals of very high frequencies, up to several hundred GHz. HBT 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 idea of using a heterojunction is as old as the conventional BJT, dating back to a patent from 1951.
The principal difference between the BJT and HBT is in the use of different semiconductor materials for the emitter and base regions, making a heterojunction. This limits 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. The high doping density reduces the base resistance while maintaining gain. The efficiency of the heterojunction is measured by the Kroemer factor.
- W. Schockley: 'Circuit Element Utilizing Semiconductive Material', United States Patent 2,569,347, 1951.
- The phototransistor effect: "The Kroemer factor is a function of the physical parameters of the materials making up the heterojunction, and can be expressed in the following way [formula given]" (The Kroemer factor was 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.)
Other webpages [change]
- NCSR HBT at the Internet Archive
- HBT Optoelectronic Circuits developed in the Technion (15Mb, 230p)
- New Material Structure Produces World's Fastest Transistor 604 GHz Early 2005