A Bipolar Junction Transistor (BJT) is a three-terminal semiconductor device that functions as an amplifier or a switch. It's one of the most important components in modern electronic circuits. There are two main types of BJTs: NPN (Negative-Positive-Negative) and PNP (Positive-Negative-Positive). The operation of a BJT is explained in terms of its three layers: the emitter, base, and collector.
Emitter (E): The emitter is heavily doped with impurities to create a majority charge carrier, either electrons (N-type) or holes (P-type). This determines whether it's an NPN or PNP transistor.
Base (B): The base is lightly doped and very thin compared to the other two layers. It's located between the emitter and collector. The flow of current through the base is what controls the larger current between the emitter and collector.
Collector (C): The collector is moderately doped and larger in size than the base. It collects the majority charge carriers that cross from the emitter due to the applied voltage.
BJTs operate based on the principles of charge carrier movement, particularly electrons and holes, in a semiconductor material such as silicon. There are two main modes of operation: active mode and cutoff mode.
Active Mode (Amplification):
In an NPN transistor:
The emitter is connected to a forward-biased voltage source (higher potential than the base).
The collector is connected to a reverse-biased voltage source (higher potential than the emitter).
The base-emitter junction is forward-biased, allowing electrons to move from the emitter to the base.
Electron flow from the emitter to the base creates a current flow across the base-emitter junction. This is called the base current (Ib).
A small base current leads to a much larger collector current (Ic) due to the transistor's current amplification properties.
The collector current is controlled by the base current through a factor called the current gain (β or hFE).
The collector current is given by the equation: Ic = β * Ib.
Cutoff Mode (Switching Off):
In an NPN transistor:
The base-emitter junction is reverse-biased, preventing significant current flow between the emitter and base.
The collector-base junction is reverse-biased as well.
With no significant base current, there's minimal collector current. The transistor is essentially off.
In summary, a BJT operates as an amplifier in active mode by allowing a small current at the base to control a larger current between the collector and emitter. In cutoff mode, the transistor is turned off, and practically no current flows between the collector and emitter. By controlling the base current, the BJT acts as a switch or an amplifier in electronic circuits, making it a fundamental component in a wide range of applications.