Transistors

Transistors have a lot of importance in the world of Physics. However, before we proceed, let’s talk about the definition of transistors. What is a transistor? It mainly refers to a device that can act as a switch or a gateway. It helps in the regulation of voltage or current flow. You’ll be surprised to know that transistors were invented at the Bell Laboratories in the year 1947 by a total of three scientists. 

A transistor is always made of layers of semiconductors. A semiconductor is a material that conducts electricity partially. They include silicon and germanium. They’re used in all transistors. 

What is a Junction Transistor? 

The Junction Transistor, also known as the Bipolar Junction Transistor (BJT), is used to amplify a signal. It is a semiconductor device that consists of two p-n junctions. It consists of three terminals: 

• Emitter: It is heavily doped. That means it has many impurities. It is not too big or not too small in size. It aids in the current flow by supplying the significant carriers.  
• Base: It is lightly doped. It forms the central segment. 
• Collector: It is bigger than the emitter. It is moderately doped. It helps supply the majority charge from the emitter and regulates the flow of current. 

Working of the BJT

The current flow occurs through the emitter, and the collector collects the current. The base is the connecting link between the emitter and the collector. In BJT, the arrows are always in the direction of the current. 

If we talk about the two types of transistors available: the p-n-p transistor and n-p-n transistor, then the current direction in the former is towards the base. However, the current direction in the latter is away from the base. The arrows point in a similar direction. It is forward-bias in base-emitter junction and reverse-bias in base-collector junction. 

Types of Bipolar Junction Transistor (BJT) 

There are two major types of transistor: 

• N-P-N Transistor 

In this transistor type, the p-type carries the minority carriers of charge. At the same time, the n-type material has the majority carrier of charges at both ends. The base comprises p-type material, and the collector and emitter are composed of n-type semiconductor material. The current flows from the emitter to the collector. It is due to a small amount of current at the terminal base. Since it is forward bias, the majority of the charge carriers are repelled away from the base. 

• P-N-P Transistor

The n-type carries the majority charge in this transistor type, and the p-type carries the minority charge. The two ends have the p-type material. There is a p-type material doped collector as well as an emitter. An n-type material base is sandwiched between the two of them. Here, the emitter is on forwarding bias, and the collector is in reverse discrimination. 

Junction Field Effect Transistor (JFET) 

The junction field-effect transistor or JFET is one of the simplest transistors available. Due to the presence of majority charge carriers, the flow of current takes place. It is as opposed to the working of a Bipolar Junction Transistor. The junction field-effect transistor works in only one direction. It was first invented in the year 1953. 

The various types of a junction field-effect transistor are: 

• n-channel JFET 
• p-channel JFET 

The only difference between these two depends on the current flow and its causes if it is due to the electrons or the holes present. 

The numerous uses of the junction field-effect transistor are: 

• As a switch.
• As a buffer. 
• As an amplifier.
• As a chopper. 

Transistor Configuration 

Some common characteristics are followed in all the transistors. They are solely based on the terminals of the transistors. The three types of configuration are as follows: 

• Common Emitter Transistor 
• Common Base Transistor 
• Common Collector Transistor 

Common Emitter Configuration

The emitter serves as the outlet for both input and output terminals. The input is obtained from the base-emitter, whereas the output is obtained from the collector-emitter. 

Herein, VCE  is the output voltage, 

VBE is the input voltage,

IC is the output current, 

IB is the input current. 

It is said that the total current is the sum of the base and collector current. 

So, IE = IB + IC

Relationship Between The Current Gains are:

Current gain (α) = IC/IE

Current gain (β) = IC/IB

Collector current IC =αIE = βIB

Input Characteristics 

The input current and voltage derive the input characteristics while the output voltage is constant. When the output voltage is changed, and the input voltage is kept constant, the input current changes at every point. That’s how the input characteristics are derived for the same. 

The input resistance can be calculated by: 

Rin = VBE/IB

Output Characteristics 

The output voltage and output current derive the output characteristics. The input current is kept constant. However, we can obtain the collector current if the base current is constant and the voltage keeps changing. 

We can calculate the output resistance as: 

Rout = VCE/IC 

Transistor as a Device 

A transistor has various applications. It can be used as a switch or an amplifier. 

1. As a Switch 

If we consider a one-sided transistor in the CE design, the input and output voltage can be calculated by applying Kirchoff’s Law: 

VBB = IBRB + VBE

VCE = VCC − ICRC

When we consider input voltage as VI    and yield voltage as Vo 

Vi = IBRB + VBE

V0 = VCC − ICRC 

1. As a Common Base Amplifier

For the calculation as a common base amplifier, 

We consider one active region of the transistor and find Vo versus Vi 

The output is: Vcc−IcRc

Vo = Vcc– IcRc

Therefore, ∆Vo = 0 – RC ∆ IC

Similarly, from Vi = IBRB + VBE

∆Vi = RB∆IB + ∆VBE

But ∆VBE is negligibly small in comparison to ∆IBRB in this circuit.

So, the voltage gain of this CE amplifier is given by: 

AV = – Rc∆ I / RB∆IB= –βac(RC/RB) 

wherein βac is equal to ∆IC/∆IB. 

Conclusion:

This article explained the basics of the transistor and its working. The junction transistor is used to amplify a signal. It is a semiconductor device that consists of two p-n junctions. It consists of three terminals: emitter, base, and collector. There are two major types of transistors, NPN transistor, and PNP transistor. A transistor can be configured in three different ways such as common emitter configuration, common base configuration, and common collector configuration.