Junction Transistor

A junction transistor, also known as a bipolar transistor, is a basic electronic device formed from semiconductor materials, and it is most commonly employed within electronic devices for signal amplification or switching. A Bipolar Junction Transistor, or BJT, is a three-terminal semiconductor device consisting of two p-n junctions and can amplify or magnify a signal. A BJT is a current-controlled device. A bipolar junction transistor is a three-terminal device. It consists of emitter, base, and collector terminals. A junction transistor uses both electrons and holes as carriers of charges.

Structure of Junction Transistor

A bipolar junction transistor (BJT) consists of three regions:

Emitter (E): Heavily doped, provides the majority of charge carriers (electrons or holes), and is small in size.

Base (B): It is extremely lightly doped and very thin. It regulates the number of carriers that reach the collector.

Collector (C): It is fairly doped and larger in terms of size to conduct heat. Accumulates the carriers from the emitter via the base

Working of Junction Transistor

Current flows from the emitter to the collector. For a correct connection between the collector and the emitter, the interaction is provided by the base for the connectivity. In a junction transistor, the arrow is directed towards the conventional current. The arrow moves away from the base in an n-p-n transistor's emitter, and the arrow moves towards the base in a p-n-p transistor's emitter. The base-emitter junction is normally forward-biased, and the base-collector junction is reverse-biased when using the junction transistor in a circuit.

Types of Junction Transistor

N-P-N Transistor:

A configuration of n-type, p-type, and n-type layers.

P-N-P transistor:

A configuration of p-type, n-type, and p-type layers.

N-P-N Transistor

In an NPN transistor, a base made of p-type semiconductor is placed between an n-type doped emitter and p-type doped collector. NPN transistors are the most utilised bipolar transistors because of the convenience of electron mobility compared to electron-hole mobility, and the simple construction and symbol of NPN transistors. In an n-p-n transistor, electrons are the majority charge carriers and holes are the minority charge carriers. A small amount of current flowing through the base terminal creates a large amount of current from emitter to collector. Due to the forward biasing, the majority of charge carriers in the emitter are repelled toward the base. The electron-hole recombination near the base is extremely minimal in the base region, and the collector region is crossed by most of the electrons.

P-N-P Transistor

In a PNP transistor, a base of n-type semiconductor is placed between a p-doped collector and a p-doped emitter. In PNP transistors, holes are the majority carriers while electrons are the minority carriers. In this, the emitter-base junction is forward-biased and the collector-base junction is reverse-biased.

Applications of Junction Transistors

Amplifiers – Radio, audio, and signal amplification
Switches – Digital electronics, logic gates
Oscillators – Used for signal generation
Voltage regulators – Power supply control
Microprocessors and CPUs – Billions of transistors per chip

Summing Up

A junction transistor is a three-layer semiconductor with three terminals: collector, base, and emitter. It is used for signal amplification or as a switch in electronic circuits. There are two types: n-p-n and p-n-p. When in operation, the emitter-base junction is forward-biased and the collector-base junction is reverse-biased. A small base current controls a large current between the collector and emitter.

Frequently Asked Questions

Q1. Which transistor type is more commonly used and why?

N-P-N transistors are utilized more frequently due to the fact that electrons (the majority carriers in n-type materials) are more mobile than holes.

Q2. Why is the base region of a transistor thin and lightly doped?

It is thin and lightly doped to minimise the recombination of charge carriers so that most carriers from the emitter reach the collector. This increases the transistor's efficiency and current gain.

Q3. What is the role of the base region?

The base is a lightly doped, thin region that governs the quantity of charge carriers transferring from the emitter to the collector.