Editing Bipolar junction transistor (section)

{{Short description|Transistor that uses both electrons and holes as charge carriers}}
{{Redirect-multi|2|BJT|Junction transistor}}
{{Use American English|date=January 2025}}
{{Use mdy dates|date=January 2025}}
{{Infobox electronic component
| name           = Bipolar junction transistor 
| image          = Transistorer (cropped).jpg
| caption        = Typical individual BJT packages. From left to right: [[SOT-23]], [[TO-92]], [[TO-126]], [[TO-3]]
| type           = Active
| inventor       = [[William Shockley]]
| invention_Year = 1948
| pins           = Base, collector, and emitter
| symbol         = [[File:NPN AND PNP BJT SYMBOLS.png|180px]]
}}

[[File:BipolarTransistor3Dmodel.png|thumb|upright|alt=box with 3 wires, one with big and silicon chip - others connect to chip with wires|3D model of a TO-92 package, commonly used for small bipolar transistors]]

A '''bipolar junction transistor''' ('''BJT''') is a type of [[transistor]] that uses both [[electron]]s and [[electron hole]]s as [[charge carrier]]s. In contrast, a unipolar transistor, such as a [[field-effect transistor]] (FET), uses only one kind of charge carrier. A bipolar transistor allows a small [[Electric current|current]] injected at one of its [[Terminal (electronics)|terminals]] to control a much larger current between the remaining two terminals, making the device capable of [[Amplifier|amplification]] or [[Electronic switch|switching]].

BJTs use two [[p–n junction]]s between two [[semiconductor]] types, n-type and p-type, which are regions in a single [[crystal]] of material. The junctions can be made in several different ways, such as changing the [[Doping (semiconductor)|doping]] of the semiconductor material as it is grown, by depositing metal pellets to form alloy junctions, or by such methods as diffusion of n-type and p-type doping substances into the crystal. The superior predictability and performance of junction transistors quickly displaced the original [[point-contact transistor]]. Diffused transistors, along with other components, are elements of [[integrated circuit]]s for analog and digital functions.  Hundreds of bipolar junction transistors can be made in one circuit at a very low cost.

Bipolar transistor integrated circuits were the main active devices of a generation of [[Mainframe computer|mainframe]] and [[minicomputer]]s, but most computer systems now use complementary metal–oxide–semiconductor ([[CMOS]]) integrated circuits relying on the field-effect transistor (FET). Bipolar transistors are still used for amplification of signals, switching, and in [[mixed-signal integrated circuit]]s using [[BiCMOS]]. Specialized types are used for high voltage and high current switches, or for [[Radio frequency|radio-frequency]] (RF) amplifiers.