Transistor basics

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

Before reading this page about how a transistor works, it is worth reading the pages entitled "What is a semiconductor" and "how a semiconductor PN junction works. These will give an understanding of some of the basic semiconductor fundamentals used in a transistor. The pages can be found on the Electronics Components menu page of the website.

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.

Semiconductor technology provides many leading edge ideas today and yet it is surprising to think that the technology is over fifty years old. 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. It is interesting to note that the inventors of the transistor were initially investigating a field effect device but could not make it work. As a result they altered the direction of their research and discovered the transistor. The FET had to wait until semiconductor material processing and other techniques had improved.

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.

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.

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.