A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. It is a solid-state device in which the current flow between two terminals (the collector and the emitter) is controlled by the amount of current that flows through a third terminal (the base) . BJTs are constructed with three doped semiconductor regions: the base, collector, and emitter, separated by two p-n junctions. There are two types of BJTs: NPN and PNP, based on the doping types of the three main terminals.
BJTs can function as a switch or an amplifier. When used as a switch, a small current input to the base controls an amplified output from the collector, making it a good switch that is controlled by its base input. When used as an amplifier, the BJT can multiply a weak input signal to about 100 times its original strength. BJTs are also used in the design of discrete and integrated circuits.
Some applications of BJTs include:
- As a detector or demodulator
- In clipping circuits to shape waves
- In logic circuits and switching circuits
While the use of BJTs has declined in favor of CMOS technology in the design of digital integrated circuits, they remain a device that excels in some applications, such as discrete circuit design.
