What are Transistors?

In this article, we are going to focus on the very basic building blocks of electronic circuits – Vacuum tubes and Transistors. Though our focus will be more on transistors.

Table of Contents

• Building Blocks of Electronics
• Types of Transistors
• Application of transistors

Building Blocks of Electronics

In electronics we need controlled flow of electrons, i.e. controlled flow of current through the electronic circuits. Devices that can allow us to have this control form the basic building blocks of electronic circuits.

Can you name a few of such devices?

Well, here are a few.

Vacuum Tubes

They were used in initial electronic circuits. Needless to say, they had many lacunae, such as:

• They were bulky.
• They consumed a lot of power. We needed high voltages to operate them – voltages in the order on 100 V.
• They were not very reliable and had very limited lifespan.

Transistors

In 1930s, scientists discovered that some solid-state semiconductors and their junctions allow us to:

• control the number of charge carriers flowing through them, and
• control the direction of flow of those charge carriers.

The number of mobile charge carriers in a semiconductor can be increased or decreased pretty easily using simple excitations like light, heat, or small applied voltage.

This was the start of modern solid-state semiconductor electronics.

Thereafter, transistors were invented in 1948, and they brought a revolution in electronics. In a transistor, p-n junctions are joined in series. It has three doped regions forming two p-n junctions between them, as shown below:

Many electronic items that we see around us, e.g. computers, calculators, etc., would not have been possible to make without these transistors.

Unlike vacuum tubes, that have been phased out and are no longer used, we still use transistors in our electronic equipments. For example, the Cathode Ray Tubes (CRT) used in earlier versions of television and computer monitors used the principle of vacuum tubes. They have now almost completely been replaced by Liquid Crystal Display (LCD) monitors with solid state electronics. So, we will focus most of our attention on Transistors.

Advantages of Transistors over Vacuum Tubes

• We know that to conduct electricity we need loose, mobile electrons. In vacuum tubes, these mobile electrons were obtained from a heated cathode and they were made to flow in an evacuated space or vacuum. So, we needed external heating mechanism and a large evacuated space. Obviously, all this made the entire setup pretty bulky. In case of transistors, it is not required – the mobile electrons are provided by the semiconductor, and they flow within the solid itself. As a result, transistors are small in size as compared to Vacuum Tubes/Valves, consume low power, and operate at low voltages.
• Transistors are more durable, i.e. they have longer life and so are more reliable. One of the main reasons for this is that they have fewer moving parts.

Types of Transistors

There are two types of transistors, based on the arrangement of semiconductors.

n-p-n transistor

In this type of transistor, n-type semiconductors (called emitter and collector) are separated by a p-type semiconductor (called base).

There are various ways in which n-p-n transistor can be represented:

p-n-p transistor

In this type of transistor, p-type semiconductors (emitter and collector) are separated by a n-type semiconductor (base).

There are various ways in which p-n-p transistor can be represented:

All the three segments of a transistor have different thickness, and their doping levels are also different.

Application of transistors

Almost all the solid-state electronic control devices nowadays are made from the bipolar or unipolar junction transistors.

Some electronic devices which use junction transistors directly are power amplifiers, voltage amplifiers, oscillators, modulators, etc.