A Bipolar Junction Transistor (BJT) is a three-terminal electronic device that amplifies and controls current. It is one of the fundamental building blocks of modern electronics and comes in two main types: NPN (Negative-Positive-Negative) and PNP (Positive-Negative-Positive). The NPN type will be used for explanation, but the principles apply to both types with appropriate polarities.
A BJT has three layers of semiconductor material, each doped differently to create two pn-junctions. These layers are:
Emitter (E): Heavily doped with a high concentration of charge carriers (either electrons or holes, depending on the type - NPN or PNP).
Base (B): Lightly doped, serving as the control region. The thickness of this layer is relatively small.
Collector (C): Moderately doped and larger in size compared to the emitter and base regions.
The operation of a BJT can be understood in two modes: active mode and cutoff mode.
Active Mode:
In active mode, the base-emitter junction is forward-biased, meaning a positive voltage is applied to the base relative to the emitter (for an NPN transistor).
This forward biasing causes electrons from the emitter to move into the base region (or holes from base to emitter in a PNP transistor).
A small number of these electrons (or holes) recombine with opposite charge carriers in the base, but a significant number diffuse through the base region and reach the collector-base junction.
The collector-base junction is reverse-biased, creating a depletion region that prevents the majority of electrons (or holes) from passing into the collector.
However, due to the minority charge carriers (holes in an NPN transistor), a small current known as the base current (Ib) flows from the emitter to the base.
The key principle of BJT operation in active mode is that the collector current (Ic) is much larger than the base current (Ib). This makes the BJT an amplifying device. The collector current is controlled by the base current, and a small change in Ib can result in a larger change in Ic.
Cutoff Mode:
In cutoff mode, both junctions (base-emitter and collector-base) are reverse-biased.
Very little current flows between the terminals, and the transistor is effectively "off."
It's important to note that a BJT is a current-controlled device. The current flowing between the collector and emitter terminals (Ic) is controlled by the small current flowing into the base terminal (Ib). By adjusting the base current, the larger collector current can be controlled, which is the basis for amplification and signal switching applications in electronic circuits.
BJTs are widely used in various applications such as amplifiers, switches, oscillators, voltage regulators, and more. They form an integral part of analog and digital circuits in electronics.