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Figure 16.7 The junction diode and its circuit symbol

The Bipolar or Junction Transistor

Construction: This is a combination of two junction diodes and consists of either a thin layer (typically 25 µm) of P-type semiconductor sandwiched between two N-type semiconductors (as shown in Figure 16.8 left) which is referred to as an N-P-N transistor, or a thin layer of N-type semiconductor sandwiched between two P-type semiconductors (as shown in Figure 16.8 right), which is referred to as a P-N-P transistor.

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Figure 16.8 The bipolar transistor

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The three regions of either type of transistor are known respectively as Collector, Base and

Emitter.

The circuit symbol for each transistor differs only in regard to the direction of the arrow between Base and Emitter.

The arrow always represents conventional current flow; thus for an N-P-N transistor it points from Base to Emitter, and for a P-N-P, from Emitter to Base.

Operation. N-P-N Transistor: If we apply an EMF across the Collector - Emitter region, as shown in Figure 16.9 left, no current flows.

However, if we now add an EMF between across the Base - Emitter region, as shown in Figure 16.9 right, a large current flows from Emitter to Collector.

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Figure 16.9 Transistor conduction

The theory governing the flow of current in a transistor is complex and generally beyond the scope of this course, but in simple terms here is what happens.

By applying an EMF, or Bias voltage, between Base and Emitter of an N-P-N transistor, the junction is forward biased and a large number of free electrons are attracted to the Base region.

However, in the relatively thin Base region, few holes are produced for these free electrons to combine with, so the surplus diffuse into the Collector region where they migrate towards the applied positive potential.

Holes that have combined with free electrons are replaced as an electron leaves the Base region for the positive terminal of the Bias supply.

Consequently, a relatively small Base - Emitter current flow produces a large Emitter - Collector flow.

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Operation. P-N-P Transistor: A P-N-P transistor’s operation is similar in all respects to that of an N-P-N transistor except that the applied EMFs are reversed.

Summary

The ability of a transistor to control a large Emitter - Collector current by means of a small Base - Emitter current means it can act as a switch or amplifier: as a switch by turning the Base - Emitter current on and off, or as an amplifier by superimposing a small alternating current signal on the Bias voltage. In conjunction with the Junction Diode and other electronic components, such as resistors, capacitors and inductors, the applications for the transistor are almost limitless.

Furthermore, the ability to control precisely those areas to which doping is applied, using photo-etching techniques, means that all of the above components can be incorporated into a highly sophisticated and complex circuit within a single, small piece of silicon. The ubiquitous computer chip is one such example.

For the future, as production techniques improve, faster, more powerful circuits will be contained in ever smaller packages, leading in turn to more sophisticated technology being incorporated in the modern airliner.

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