A Bipolar Junction Transistor (BJT) is a three-layer semiconductor device that can amplify or switch electrical signals. It's a type of transistor that relies on the movement of both majority and minority charge carriers within its structure. There are two main types of BJTs: NPN (negative-positive-negative) and PNP (positive-negative-positive), which differ in terms of the doping of their three layers.
The three layers of a BJT are called the emitter, base, and collector. Each layer is made of a different type of semiconductor material, typically either N-type or P-type. Here's a breakdown of their operation:
Emitter (E): This layer is heavily doped with either electrons (N-type) or holes (P-type). It serves as the source of majority charge carriers that will flow into the base region.
Base (B): This layer is lightly doped and physically sandwiched between the emitter and collector. The base controls the flow of majority charge carriers from the emitter to the collector. It's an important part of the transistor's operation because it determines the amount of current flowing through the transistor.
Collector (C): This layer is also heavily doped, but with the opposite type of carriers compared to the emitter. If the emitter is N-type, the collector is P-type (and vice versa). The collector collects the majority charge carriers (electrons or holes) that flow through the base region.
The operation of a BJT involves two main modes: active mode and cutoff/saturation mode.
Active Mode Operation:
In the active mode, the emitter-base junction is forward-biased (meaning the emitter is more positive than the base) while the collector-base junction is reverse-biased (meaning the collector is more positive than the base). Here's what happens:
Emitter Injection: Electrons (or holes) from the emitter region are injected into the base region due to the forward bias. These are the minority charge carriers in the emitter region.
Base Transit: The injected minority carriers cross the base region and reach the collector-base junction. Only a small fraction of the injected carriers recombine in the base; the rest cross the base and enter the collector.
Collector Collection: Majority carriers (electrons or holes) from the collector region are collected at the collector terminal. This forms the main output current, called the collector current (IC), which is proportional to the base current (IB).
Cutoff/Saturation Mode Operation:
In the cutoff mode, the transistor is effectively off. No current flows between the collector and emitter, and both junctions are biased in a way that prevents current flow. In the saturation mode, the transistor is fully on, and the collector-emitter voltage is very small. In this state, the transistor acts as a low-resistance switch.
In summary, a BJT operates by controlling the flow of minority carriers from the emitter to the collector region through the base region. By varying the base current, you can control a much larger collector current, allowing the BJT to amplify signals or function as a switch in electronic circuits.