A Bipolar Junction Transistor (BJT) is a type of semiconductor device that is widely used in electronic circuits for amplification or switching purposes. It consists of three layers of semiconductor material: the emitter, base, and collector, arranged in two types of configurations: NPN (Negative-Positive-Negative) and PNP (Positive-Negative-Positive).
There are two main modes of operation for a BJT: active mode and saturation mode. Additionally, there is a third mode called cut-off mode, where the transistor is essentially turned off.
Active Mode: In the active mode, the BJT is biased in such a way that the base-emitter junction is forward-biased (meaning the base is more positive than the emitter in an NPN transistor) and the base-collector junction is reverse-biased. This arrangement allows a small base current to control a larger collector current. In this mode, the transistor acts as an amplifier, where a small input current or voltage signal at the base controls the larger output current at the collector. The transistor is in its linear region, and changes in the base current result in proportional changes in the collector current.
Saturation Mode: In the saturation mode, the base-emitter junction is still forward-biased, but the base-collector junction is also forward-biased. This configuration results in the transistor acting as a closed switch, allowing a large current to flow from the collector to the emitter. In this mode, the transistor is fully conducting, and there is minimal voltage drop across the collector-emitter junction. Saturation mode is often used in digital switching applications.
Cut-off Mode: In the cut-off mode, both the base-emitter and base-collector junctions are reverse-biased, preventing any significant current flow between the collector and the emitter. The transistor is effectively turned off in this mode, acting as an open switch.
The behavior and characteristics of a BJT can be described using various parameters like current gain (β or hFE), voltage ratings, and more. NPN and PNP transistors exhibit similar behavior but with reversed current polarities due to the differences in the doping of the layers.
It's important to note that while BJTs have been foundational components in electronics, they have been partly supplanted by Field-Effect Transistors (FETs) in certain applications due to their advantages like higher input impedance and better thermal stability.