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 is classified into two types: NPN and PNP. The operation of a BJT relies on the movement of charge carriers—electrons and holes—across these layers, allowing it to control current flow. BJTs are widely used in various electronic applications, including amplifiers, oscillators, and switching circuits. They are essential components in many devices, such as computers, radios, and televisions, due to their ability to manage electrical signals effectively.