What is Heterojunction bipolar transistor?
The '''heterojunction bipolar transistor''' ('''HBT''') is a type of [[bipolar junction transistor]] (BJT) which uses different [[semiconductor]] materials for the [[transistor|emitter]] and [[transistor|base]] regions, making a [[heterojunction]]. The HBT can handle signals of much higher frequencies, (up to several hundred [[gigahertz|GHz]]) than BJT. HBT is commonly used in modern ultrafast circuits, mostly [[Radio frequency|radio-frequency]] (RF) systems, and in applications requiring a high power efficiency, such as RF power amplifiers in [[mobile phone]]s. The idea of using a heterojunction is as old as the conventional BJT, dating back to a patent from 1951.<ref>W. Schockley: 'Circuit Element Utilizing Semiconductive Material', United States Patent 2,569,347, 1951.</ref> ==Materials== [[Image:Pnp Heterostructure Bands.png|thumb|Bands in graded heterojunction npn bipolar transistor. Barriers indicated for electrons to move from emitter to base, and for holes to be injected backward from base to emitter; Also, grading of [[band gap]] in base assists electron transport in base region; Light colors indicate [[depletion region|depleted regions]] ]] 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 energy|potential]] barrier in the [[valence band]] is higher than in the [[conduction band]]. Unlike BJT technology, this allows a high [[Doping (semiconductor)|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. ==References== <references/> ==Other websites== * {{Wayback|url=http://www.onr.navy.mil/sci_tech/31/312/ncsr/devices/hbt.asp|title=NCSR HBT|date=20080404224041}} * [http://www.ee.technion.ac.il/HighSpeedElectronics/Publications/Shraga_Kraus_Thesis.pdf HBT Opto