A BJT, or Bipolar Junction Transistor, is a transistor that can act as a switch, amplifier, or oscillator. Its operation relies on two types of charge carriers: holes and electrons.

Key Characteristics

• “Bipolar” Nature: The name “bipolar” comes from the fact that its operation involves two types of charge carriers—holes (positive charges) and electrons (negative charges).
• Core Function: A BJT is capable of current amplification, where a small current applied to its base can control a much larger current flowing through its other terminals (a characteristic known as current gain, or β).
• Terminals: It has three terminals, known as the emitter, the base, and the collector.

From Amplifier to Switch

In addition to amplifying signals, a BJT is an effective electronic switch. As a switch, the BJT is controlled by the current flowing into its base. By applying a small forward-bias voltage to the base-emitter junction to allow a small base current to flow, a much larger current is permitted to flow between the collector and the emitter. This ability to control a large current with a small one is fundamental to many electronic circuits.

While the BJT provides excellent current amplification, controlling it requires a continuous base current, which can be inefficient. This limitation drove the development of a hybrid device that combines the BJT’s high-power output with a more energy-efficient, voltage-controlled input—the IGBT.