How Does a Transistor Work

- an overview, summary or tutorial about the basics of how a transistor works, and transistor theory.

Transistor Tutorial Includes:
Transistor basics     Transistor specifications     Transistor and diode numbering codes     Choosing replacement transistors    

Transistors are at the very core of today's electronics technology. The development of the transistor has resulted in many changes to the world. It has resulted in everything from portable transistor radios, through to cellular phones, and computers. All these and many more everyday items have all been made possible by the invention of the transistor. All these developments have taken place since the development of the first transistor.

Short transistor history

Semiconductor technology is now well established but it has been used for over a hundred years. The first semiconductor effects were noticed back in the early 1900s when the first wireless or radio sets were being used. Various ideas were being investigated as detectors. Thermionic valve or vacuum tube technology was introduced in 1904, but these devices were expensive, and also required powering by a battery. Soon afterwards the Cat's Whisker detector was discovered. This consisted of a thin wire placed onto one of a number of types of material. Today we know these were all semiconductors. As a result these created the first diodes.

Other semiconductor effects were also discovered. In 1908 H J Round investigating Cat's Whisker diodes discovered that when a current was passed through a detector using a particular material, light was emitted. He reported the fact in a magazine of the day, but little was done.

During the 1920s and 1930s much of the work undertaken into electronics devices was focussed on thermionic valve or vacuum tube technology. However there was also some original work being undertaken into particle physics. Although not directed towards electronics technology, this was to provide the foundations upon which semiconductor technology would be built.

With the development of radar in World War II, there was a growing need for high performance microwave components. Detectors were a particular need. It was soon realised that developments of what was effectively a cat's Whisker performed well at these frequencies, and using the early work done into semiconductor physics, these diodes were developed further as point contact diodes.

After the war, much research continued. In particular Bell Labs in the USA invested heavily, and one of their groups with three engineers, Bardeen, Brattain, and Shockley stated to investigate a three terminal device in which the voltage on one terminal would affect the current flow through a semiconductor path - effectively a field effect device. The device did not work and therefore they turned their research towards another idea. We now know this device as the bipolar transistor.

The transistor effect was first observed on 23rd December 1947 at Bell Labs in the USA, and it was then demonstrated to senior management of the company on Christmas Eve. Little did they know what an impact the device would have on the lives of billions of people around the world.

The original idea they were investigating - the field effect transistor had to wait until semiconductor material technology had advanced and allowed the semiconductors to be refined further and processed more accurately before it could be made to work. Nevertheless the foundations that enabled semiconductor had now taken place, enabling semiconductors to become the dominant technology ousting tubes or valves from a place they had occupied for around 50 years.

Basic transistor structure

The transistor is a three terminal device and consists of three distinct layers. Two of them are doped to give one type of semiconductor and the there is the opposite type, i.e. two may be n-type and one p-type, or two may be p-type and one may be n-type.. They are arranged so that the two similar layers of the transistor sandwich the layer of the opposite type. As a result transistor are designated either P-N-P (PNP) types of N-P-N (NPN) types according to the way they are made up.

Transistor diagrammatic structure and circuit symbols
Transistor diagrammatic structure and circuit symbols

The centre region is called the base and gains its name from the fact that in the very earliest transistors it formed the "base" for the whole structure. The other two connections are called the emitter and collector. These names result from the way in which they either emit or collect the charge carriers. It is also essential that the base region is very thin if the device is to be able to operate. In today's transistors the base may typically be only about 1 mm [micrometre] across. It is the fact that the base region of the transistor is thin that is the key to the operation of the device

Transistor operation

A transistor can be considered as two P-N junctions placed back to back. One of these, namely the base emitter junction is forward biased, whilst the other, the base collector junction is reverse biased. It is found that when a current is made to flow in the base emitter junction a larger current flows in the collector circuit even though the base collector junction is reverse biased.

For clarity the example of an NPN transistor is taken. The same reasoning can be used for a PNP device, except that holes are the majority carriers instead of electrons.

When current flows through the base emitter junction, electrons leave the emitter and flow into the base. However the doping in this region is kept low and there are comparatively few holes available for recombination. As a result most of the electrons are able to flow right through the base region and on into the collector region, attracted by the positive potential.

How does a transistor work
Diagram shwoing how a transistor works

Only a small proportion of the electrons from the emitter combine with holes in the base region giving rise to a current in the base-emitter circuit. This means that the collector current is much higher. The ratio between the collector current and the base current is given the Greek symbol b. For most small signal transistors this may be in the region 50 to 500. In some cases it can be even higher. This means that the collector current is typically between 50 and 500 times that flowing in the base. For a high power transistor the value of b is somewhat less: 20 is a fairly typical value.

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