BASIC ELECTRONICS TRANSISTOR

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

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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.