Bipolar Junction Transistor

A bipolar junction transistor (BJT) is a type of transistor. BJTs use two p–n junctions between two semiconductor types, n-type and p-type. The BJT is a semiconductor device commonly used for amplification as well as switching. The BJT consists of three regions: the emitter, base, and collector, with two PN junctions separating them.

In operation, the BJT can be in one of two modes: the active mode, where it amplifies signals, or the cutoff or saturation mode, where it acts as a switch. The flow of current between the emitter and collector is controlled by the relatively small current flowing into the base terminal, allowing for significant signal amplification or switching capability. Its three-layer structure and ability to control a large amount of current with a small input make it a fundamental component in many electronic circuits.