A Bipolar Junction Transistor (BJT) is a type of semiconductor device that can amplify or switch electronic signals. It consists of three layers of semiconductor material, typically made of silicon, and has three terminals: the emitter, base, and collector. The operation of a BJT relies on the movement of charge carriers, which can be either electrons or holes, allowing it to control larger currents with smaller input signals. BJTs are classified into two main types: NPN and PNP, based on the arrangement of the semiconductor layers. In an NPN transistor, the flow of electrons from the emitter to the collector is controlled by the base current, while in a PNP transistor, holes flow in the opposite direction. BJTs are widely used in various electronic applications, including amplifiers, oscillators, and switching circuits.