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 current flow. BJTs come in two main types: NPN and PNP. In an NPN transistor, a small current at the base terminal controls a larger current flowing from the collector to the emitter. Conversely, in a PNP transistor, the current flows in the opposite direction. These properties make BJTs essential components in various electronic circuits, including amplifiers and switches.