A Bipolar Junction Transistor (BJT) is a three-terminal semiconductor device that can be used as an amplifier, a switch, or in various electronic circuits. It comes in two major types: NPN (Negative-Positive-Negative) and PNP (Positive-Negative-Positive), with the distinction based on the arrangement of the semiconductor layers.
Here's a breakdown of the operation of an NPN BJT, which is the more common type:
Structure: A BJT consists of three semiconductor regions, typically made of silicon, which are referred to as the emitter (E), base (B), and collector (C). These regions are doped differently to create the necessary conductivity differences. The emitter is heavily doped (either with extra electrons for NPN or holes for PNP), the base is lightly doped, and the collector is moderately doped.
Biasing: Biasing is the process of applying suitable voltages to the emitter and base terminals to establish a forward-biased emitter-base junction and a reverse-biased base-collector junction. In the NPN transistor:
The emitter terminal is connected to a more positive voltage (often ground or the lowest potential in the circuit).
The base terminal is connected to a more negative voltage compared to the emitter (typically a few hundred millivolts to establish a forward bias).
Modes of Operation:
Active Mode: This is the region where the transistor generally operates. A small current flows from the emitter to the base, creating a larger current flow from the collector to the emitter. The emitter current is the sum of the base current and the collector current.
Saturation Mode: In this mode, the transistor is fully turned on, allowing a high collector current to flow without much restriction. It's typically used when the transistor acts as a switch.
Cutoff Mode: In this mode, the transistor is turned off. No current flows from the collector to the emitter, and it acts as an open switch.
Transistor Action: The operation of a BJT is primarily based on the behavior of the two back-to-back pn-junctions (emitter-base and base-collector). In the NPN transistor:
Electrons flow from the heavily-doped emitter to the base, which is lightly doped. This constitutes the base current (Ib).
A small fraction of these electrons diffuse across the thin base region and reach the collector, creating the collector current (Ic).
The emitter current (Ie) is the sum of the base and collector currents: Ie = Ib + Ic.
Current Gain (Beta, β): The transistor's ability to amplify current is quantified by the current gain (β), also known as the "hFE" parameter. It's the ratio of collector current to base current (β = Ic / Ib) and is typically in the range of 20 to 1000 for common BJTs.
In summary, a BJT operates as an amplifier by using a small current at the base terminal to control a much larger current between the collector and emitter terminals. This controlled amplification of current forms the basis for its use in various electronic circuits and applications.