Transistor

What is a transistor? A transistor is a type of semiconductor device that can be used to conduct and insulate electric current or voltage. A transistor basically acts as a switch and an amplifier. In simple words, we can say that a transistor is a miniature device that is used to control or regulate the flow of electronic signals.

Transistors are one of the key components in most of the electronic devices that are present today. Developed in the year 1947 by three American physicists, John Bardeen, Walter Brattain and William Shockley, the transistor is considered one of the most important inventions in the history of science.

Table of Contents

• Parts of a Transistor
• Types of Transistors
• How Do Transistors Work?
• Characteristics of Transistor
• Advantages of Transistor

Parts of a Transistor

A typical transistor is composed of three layers of semiconductor materials or, more specifically, terminals which help to make a connection to an external circuit and carry the current. A voltage or current that is applied to any one pair of the terminals of a transistor controls the current through the other pair of terminals. There are three terminals for a transistor. They are listed below:

• Base: This is used to activate the transistor.
• Collector: It is the positive lead of the transistor.
• Emitter: It is the negative lead of the transistor.

Well, the very basic working principle of a transistor is based on controlling the flow of current through one channel by varying the intensity of a smaller current that is flowing through a second channel.

Also Read:

• Transistor as a Switch
• Transistor as an Amplifier

Types of Transistors

There are mainly two types of transistors, based on how they are used in a circuit.

Bipolar Junction Transistor (BJT)

The three terminals of BJT are the base, emitter and collector. A very small current flowing between the base and emitter can control a larger flow of current between the collector and emitter terminal.

Furthermore, there are two types of BJT, and they include:

• P-N-P Transistor: It is a type of BJT where one n-type material is introduced or placed between two p-type materials. In such a configuration, the device will control the flow of current. PNP transistor consists of 2 crystal diodes which are connected in series. The right side and left side of the diodes are known as the collector-base diode and emitter-base diode, respectively.

• N-P-N Transistor: In this transistor, we will find one p-type material that is present between two n-type materials. N-P-N transistor is basically used to amplify weak signals to strong signals. In an NPN transistor, the electrons move from the emitter to the collector region, resulting in the formation of current in the transistor. This transistor is widely used in the circuit.

There are three types of configuration, which are common base (CB), common collector (CC) and common emitter (CE).

In common base (CB) configuration, the base terminal of the transistor is common between input and output terminals.

In common collector (CC) configuration, the collector terminals are common between the input and output terminals.

In common emitter (CE) configuration, the emitter terminal is common between the input and the output terminals.

Field Effect Transistor (FET)

For FET, the three terminals are Gate, Source and Drain. The voltage at the gate terminal can control a current between the source and the drain. FET is a unipolar transistor in which N-channel FET or P-channel FET are used for conduction. The main applications of FETs are in low noise amplifiers, buffer amplifiers and analogue switches.

Other Types

Apart from these, there are many other types of transistors which include MOSFET, JFET, insulated-gate bipolar transistor, thin-film transistor, high electron mobility transistor, inverted-T field-effect transistor (ITFET), fast-reverse epitaxial diode field-effect transistor (FREDFET),  Schottky transistor, tunnel field-effect transistor, organic field-effect transistor (OFET), diffusion transistor, etc.

How Do Transistors Work?

Let us look at the working of transistors. We know that BJT consists of three terminals (Emitter, Base and Collector). It is a current-driven device where two P-N junctions exist within a BJT.

One P-N junction exists between the emitter and base region, and the second junction exists between the collector and base region. A very small amount of current flow through the emitter to the base can control a reasonably large amount of current flow through the device from the emitter to the collector.

In the usual operation of BJT, the base-emitter junction is forward-biased, and the base-collector junction is reverse-biased. When a current flows through the base-emitter junction, the current will flow in the collector circuit.

In order to explain the working of the transistor, let us take an example of an NPN transistor. The same principles are used for the PNP transistor, except that the current carriers are holes, and the voltages are reversed.

Operation of NPN Transistor

The emitter of the NPN device is made of n-type material; hence, the majority of carriers are electrons. When the base-emitter junction is forward-biased, the electrons will move from the n-type region towards the p-type region, and the minority carrier holes move towards the n-type region.

When they meet each other, they will combine, enabling a current to flow across the junction. When the junction is reverse-biased, the holes and electrons move away from the junction, and now, the depletion region forms between the two areas and no current will flow through it.

When a current flows between the base and emitter, the electrons will leave the emitter and flow into the base, as shown above. Normally, the electrons will combine when they reach the depletion region.

But the doping level in this region is very low, and the base is also very thin. This means that most of the electrons are able to travel across the region without recombining with holes. As a result, the electrons will drift towards the collector.

In this way, they are able to flow across what is effectively a reverse-biased junction, and the current flows in the collector circuit.

Also Read: p-n Junction

Characteristics of Transistor

Characteristics of the transistor are the plots which can represent the relation between the current and the voltage of a transistor in a particular configuration.

There are two types of characteristics.

• Input characteristics: It will give us the details about the change in input current with the variation in input voltage by keeping output voltage constant.
• Output characteristics: It is a plot of output current with output voltage by keeping the input current constant.
• Current transfer characteristics: This plot shows the variation of the output current with the input current by keeping the voltage constant.

Input Characteristics

CB Configuration

The following chart will describe the variation of emitter current, I_E with base – Emitter voltage, V_BE keeping collector voltage constant, V_CB.

CC Configuration

It shows the variation in I_B in accordance with V_CB with collector-emitter voltage V_CE keeping constant.

CE Configuration

Here it shows the variation in I_B in accordance with V_BE by keeping V_CE constant.

Output Characteristics

CB Configuration

This chart shows the variation of collector current, I_C with V_CB, by keeping the emitter current I_E constant.

CC Configuration

This exhibits the variation in I_E against the changes in V_CE by keeping I_B constant.

CE Configuration

Here, it shows the variation in I_C with the changes in V_CE by keeping I_B constant.

Current Transfer Characteristics

CB Configuration

It gives the variation of I_C with the I_E by keeping V_CB as constant.

CC Configuration

This shows the variation of I_E with I_B by keeping V_CE constant.

CE Configuration

Here, it shows the variation of I_C with I_B by keeping V_CE constant.

Advantages of Transistor

• Lower cost and smaller in size.
• Smaller mechanical sensitivity.
• Low operating voltage.
• Extremely long life.
• No power consumption.
• Fast switching.
• Better efficiency circuits can be developed.
• Used to develop a single integrated circuit.

Limitations of Transistors

Transistors have a few limitations, and they are as follows:

• Transistors lack higher electron mobility.
• Transistors can be easily damaged when electrical and thermal events arise. For example, electrostatic discharge in handling.
• Transistors are affected by cosmic rays and radiation.

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