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 are electrons and holes, allowing it to control current flow. BJTs are classified into two types: NPN and PNP, based on the arrangement of the semiconductor layers. In an NPN transistor, the flow of current is primarily due to electrons, while in a PNP transistor, it is due to holes. BJTs are widely used in various electronic applications, including amplifiers and switching circuits.