Bipolar Junction Transistors (BJT) and Their Working

Definition

A bipolar junction transistor (BJT) is a three-layer, three-terminal semiconductor device formed by joining two p-n junctions in such a way that it can control the flow of current through the use of a small input current.

The three terminals are:

Emitter (E)

Base (B)

Collector (C)

Depending on the arrangement of semiconductor layers, BJTs are of two types:

NPN transistor

PNP transistor

A BJT operates by using a small base current to control a much larger collector current, making it a current-controlled device.

Main Content

1. Structure and Types of BJT

A BJT consists of three semiconductor regions arranged as either NPN or PNP, creating two p-n junctions: the emitter-base junction and the collector-base junction.
The emitter is heavily doped to inject a large number of charge carriers, the base is very thin and lightly doped to allow carrier movement with minimal recombination, and the collector is moderately doped and physically larger to collect carriers and dissipate heat.

In an NPN transistor, two n-type regions are separated by a p-type base. Electrons are the majority carriers and are mainly responsible for current conduction. In a PNP transistor, two p-type regions are separated by an n-type base, and holes are the majority carriers. NPN transistors are generally more widely used because electrons have higher mobility than holes, giving better performance in many applications.

The physical design of the transistor is very important for its operation. The emitter is designed to inject carriers efficiently, the base is made thin so that most injected carriers can pass through it, and the collector is designed to safely collect carriers and withstand larger voltages. This layered structure is what enables transistor action.

2. Current Flow and Transistor Action

A BJT works by applying a small current to the base-emitter junction, which allows a much larger current to flow from collector to emitter.
The relationship among the three currents is expressed as Ie = Ib + Ic, where emitter current equals the sum of base current and collector current.

When the emitter-base junction is forward biased, carriers are injected from the emitter into the base. Since the base is very thin and lightly doped, only a small portion of these carriers recombine in the base region. Most carriers cross the base and are attracted into the collector region by the reverse-biased collector-base junction. This produces a large collector current.

This mechanism is known as transistor action. The small base current controls the much larger collector current, allowing the transistor to function as an amplifier or switch. The current gain of the transistor is an important feature. In common-emitter configuration, the current gain is represented by β (beta) and is defined as Ic/Ib. A small increase in base current can result in a large increase in collector current.

For example, if a transistor has a beta value of 100, a base current of 20 μA can control a collector current of about 2 mA. This demonstrates the amplifying property of the BJT.

3. Operating Regions and Configurations

A BJT can operate in different regions such as cutoff, active, saturation, and reverse active, depending on the biasing of its junctions.
The three main configurations of BJT are common base, common emitter, and common collector, each having different voltage, current, and power characteristics.

In the cutoff region, both junctions are reverse biased, and the transistor is OFF. Very little current flows, so this region is used in switching applications to represent logic 0 or open switch conditions.

In the active region, the emitter-base junction is forward biased and the collector-base junction is reverse biased. This is the normal operating region for amplification. The collector current is controlled by the base current and is nearly independent of collector voltage within limits.

In the saturation region, both junctions are forward biased. The transistor is fully ON, and collector current is maximum for the given circuit. This region is also used in switching circuits.

The common emitter configuration is the most widely used because it gives both current gain and voltage gain. The common base configuration has low input resistance and is useful in high-frequency circuits. The common collector configuration, also called an emitter follower, provides high input resistance and low output resistance, making it useful for impedance matching.

Working / Process

1. Bias the transistor correctly

In an NPN transistor, the base-emitter junction is forward biased and the collector-base junction is reverse biased for normal active operation. In a PNP transistor, the polarity is reversed.
This biasing prepares the transistor to control current flow properly.

2. Inject carriers from the emitter into the base

The forward-biased emitter junction causes majority carriers from the emitter to move into the base region.
Because the base is very thin and lightly doped, only a small number of these carriers recombine with base carriers, producing a small base current.

3. Collect most carriers at the collector and obtain output current

The reverse-biased collector junction attracts the remaining carriers into the collector region.
This results in a large collector current controlled by the small base current, allowing the transistor to amplify signals or act as an electronic switch.

In an NPN transistor, electrons flow from emitter to collector, while conventional current flows from collector to emitter. In a PNP transistor, holes flow from emitter to collector, with current directions reversed. The exact carrier motion depends on transistor type, but the working principle remains the same: a small input current controls a larger output current.

Advantages / Applications

High current gain and amplification capability

: A BJT can amplify weak electrical signals into stronger ones, making it highly useful in audio amplifiers, sensor circuits, and communication systems.

Reliable switching action

: BJTs can operate as fast electronic switches in digital logic circuits, relay drivers, motor control circuits, and power switching applications.

Wide range of practical uses

: They are used in oscillators, voltage regulators, current mirrors, signal processing circuits, wave-shaping circuits, and integrated circuits due to their versatility and good performance.

BJTs are also favored in many analog applications because of their predictable current behavior and ability to operate in the active region with good linearity. Their use in small-signal and power applications makes them one of the most important semiconductor devices in electronics.

Summary

BJT is a three-terminal semiconductor device used for amplification and switching.
Its operation depends on the movement of both electrons and holes.
A small base current controls a much larger collector current.

Important terms to remember

Emitter
Base
Collector
NPN
PNP
Biasing
Active region
Cutoff region
Saturation region
Current gain