What is a Transistor? | Working Principles

In this article, we're going to introduce you to an electronic component called the Transistor.
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In this article, we’re going to introduce you to an electronic component called the Transistor.

Believe it or not, your phone has hundreds of thousands of transistors in it. Your computer has millions! It’s not an exaggeration to say that life would be much different today if the transistor hadn’t been invented.

The name comes from the merging of the two words transfer and resistor to become transfer-resistor.

Shorten up the 2 words and you get transistor.

So, the name infers that the transistor somehow performs some sort of transfer of resistance. We’ll look a bit closer at that concept later.

A transistor is an electronic component found in a variety of circuits and is used to amplify or switch electronic signals and electrical power.

What is a Transistor

Types of transistors

There are many different transistor types each with its own electronic symbol.

To name just a few:

– There’s the BJT or Bipolar Junction Transistor

– Another common transistor is the FET or Field Effect Transistor

– There’s also the UJT or Unijunction Transistor

Different Types of Transistors

What is a semiconductor?

A transistor is a semiconductor device.

OK….so what is a semiconductor?

Well, in simple terms, a semiconductor is not a good conductor, but neither is it a good insulator. It is somewhere in between.

Everybody has heard of Silicon Valley.

Well, Silicon is a semiconductor, and Silicon Valley is the home of a large number of inventors and manufacturers specializing in silicon-based transistors and integrated circuit chips.

The majority of transistors are made of silicon. A small percentage of transistors are made from germanium, which is another semiconductor material.

What is a transistor made of

What do the letters N and P mean?

A basic transistor consists of 3 chunks, or pieces of silicon sandwiched together.

As mentioned earlier there are many different types of transistors.

In this article, we’ll focus on the Bipolar Junction Transistor which is probably the most common.

A good question to ask now is What do the letters N and P mean?

The first stage of making a transistor is the process of changing the semiconductor conducting properties by introducing impurities into its structure. This conduction change process is referred to as Doping.

Simply stated, a P slice of the sandwich is more positive and an N slice of the sandwich is more negative because of Doping.

Transistor Construction

Bipolar Junction Transistor

Ok….now we know a transistor is basically a sandwich made of 3 chunks of a semiconductor material doped to make the P chunks more positive, and the N chunks more negative.

Let’s take a closer look at a BJT.

There are 2 types of BJT’s. They are given names based on the doping content of the semiconductor chunks in each.

One is called an NPN and the other is called a PNP. Each has its own electronic symbol.

There is a terminal connected to each chunk of the sandwich and each terminal is given a name. The names are Emitter, Base, and Collector.

The Bipolar Junction Transistor

We are purposely avoiding transistor theory including the concepts of covalent bonding, the depletion zone, and biasing as there are an infinite number of websites where you can get that information if you are interested.

Here’s a couple of tips for you….

The arrow is always part of the Emitter/Base connection.

The types can be identified by the direction of the arrow.

Transistor current direction

Transistor applications

Transistor as an amplifier

If we look at controlling a large voltage with a small voltage, we can say that we are performing an amplification. A transistor can do that.

Transistor Amplifiers

Transistor as a switch

The transistor’s ability to act as a switch or perform a transfer of resistance makes it a very useful component in industrial applications.

Let’s look at how a transistor works as a switch.

The switch part of the transistor is between the Collector and the Emitter.

The switch is operated by changing the voltage between the Base and the Emitter.

If the input voltage is 0 volts, the switch is open, the resistance is infinite, and the output voltage is +10 volts.

If the input voltage is +10 volts, the switch is closed, the resistance is zero, and the output voltage is 0 volts.

-Working of Transistor as a Switch

Transistor radio

There are countless transistor applications.

One application that had a huge impact was the invention of the Transistor radio.

Before the advent of transistors, radios were large bulky pieces of furniture filled with vacuum tubes providing the required audio amplification.

After the invention of the transistor, audio signals could now be performed by tiny transistors.

So, the transistor radio became portable and quite small.

Transistor Applications.

Transistor application in industry

Proximity switch

Transistors are used in industry as well.

For example, traditional limit switches are being replaced with active proximity sensors.

The output of an active proximity sensor is a transistor switch. With no moving parts and nothing to wear out or breakdown, the active proximity switch is the hands-down winner over a mechanical limit switch.

Uses of Transistor and its application in industry

PLC output modules

Incorporating transistors into PLC output modules is another example of where transistors are used in the industry.

PLC output modules are now built with transistor output circuits.

Early PLCs utilized relay switching to operate loads.

Instead of operating a relay, a PLC module can control the output device with a transistor switch. Again….no moving parts….better reliability and a definite advantage in switching speed!

Transistor PLC Output


Ok, …let’s review what we’ve covered in this article

– Transistors are electronic components used in a circuit to control a large current or voltage with a small voltage or current.

– The word Transistor comes from combining the 2 words Transfer and Resistor.

– Transistors are made of silicon or germanium which are semiconductor materials.

– A semiconductor is neither a conductor nor an insulator, but somewhere in between.

– The most common transistors are the BJT.

– The two types of BJT are NPN and PNP

– BJT’s are used for audio amplification and as electronic switching devices.

You might want to review our other articles:

3-wire Inductive Proximity Sensor | How to Read the Datasheet

Limit Switch Explained | Working Principles

How to Wire Discrete DC Sensors to PLC – Part 2

If you have any questions about transistors and the applications of transistors in the industry, add them in the comments below and we will get back to you in less than 24 hours.

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The RealPars Team

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