Editing Bipolar junction transistor (section)

== Applications ==
The BJT remains a device that excels in some applications, such as discrete circuit design, due to the very wide selection of BJT types available, and because of its high transconductance and output resistance compared to [[MOSFET]]s.

The BJT is also the choice for demanding analog circuits, especially for [[very-high-frequency]] applications, such as [[radio-frequency]] circuits for wireless systems.

=== High-speed digital logic ===
[[Emitter-coupled logic]] (ECL) use BJTs.

Bipolar transistors can be combined with MOSFETs in an integrated circuit by using a BiCMOS process of wafer fabrication to create circuits that take advantage of the application strengths of both types of transistor.

=== Amplifiers ===
{{Main|Electronic amplifier}}

One of the most prominent early uses of the transistor was in consumer products such as the [[transistor radio]] which [[Regency TR-1|began production in 1954]]. The use of transistors in handheld radios and would also jumpstart a small Japanese company named Tokyo Tsushin Kogyo K.K. to prominence with its [[TR-55]] transistor radio bearing the name the company would soon change to match: [[Sony]]. The follow-on pocket-sized Sony TR-63 and several larger models by other manufacturers cemented the transistor and miniaturized electronics as critical to the new, portable consumer device market for decades to come.{{cn|reason=Need RS for "cemented the transistor and miniaturized electronics as critical to the new, portable consumer device market for decades to come"|date=June 2025}}

The [[#Transistor parameters: alpha (α) and beta (β)|transistor parameters]] α and β characterize the [[gain (electronics)|current gain]] of the BJT. It is this gain that allows BJTs to be used as the building blocks of electronic amplifiers. The three main BJT amplifier topologies are:
* [[Common emitter]]
* [[Common base]]
* [[Common collector]]

=== Temperature sensors ===
{{Main|Silicon bandgap temperature sensor}}

Because of the known temperature and current dependence of the forward-biased base–emitter junction voltage, the BJT can be used to measure temperature by subtracting two voltages at two different bias currents in a known ratio.<ref>{{Cite web |url=https://www.analog.com/en/resources/design-notes/ic-temperature-sensors-find-the-hot-spots.html |title=IC Temperature Sensors Find the Hot Spots |date=February 21, 2002 |website=analog.com |access-date=January 12, 2025 }}</ref>

=== Logarithmic converters ===
Because base–emitter voltage varies as the logarithm of the base–emitter and collector–emitter currents, a BJT can also be used to compute [[logarithm]]s and anti-logarithms. A diode can also perform these nonlinear functions but the transistor provides more circuit flexibility.

=== Avalanche pulse generators ===
Transistors may be deliberately made with a lower collector to emitter breakdown voltage than the collector to base breakdown voltage. If the emitter–base junction is reverse biased the collector emitter voltage may be maintained at a voltage just below breakdown. As soon as the base voltage is allowed to rise, and current flows [[Avalanche breakdown|avalanche]] occurs and impact ionization in the collector base depletion region rapidly floods the base with carriers and turns the transistor fully on. So long as the pulses are short enough and infrequent enough that the device is not damaged, this effect can be used to create very sharp falling edges.

Special [[avalanche transistor]] devices are made for this application.