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, which are called the emitter, base, and collector. The BJT can be classified into two types: NPN and PNP, depending on the arrangement of the semiconductor layers. The operation of a BJT relies on the movement of charge carriers, which are electrons and holes. When a small current flows into the base, it allows a larger current to flow from the collector to the emitter, enabling the transistor to control electrical signals effectively. This property makes BJTs essential components in various electronic circuits.