Question

Describe the basic operation of a transistor

Answer 1

The transistor can be considered as two p-n junctions that are placed back to back. The structure has two PN junctions with a narrow base region between the two outlying areas for the collector and emitter.

In normal operation, the base emitter junction is forward biased and the base collector junction is reverse biased. When a current flows through the base emitter junction, a current also flows in the collector circuit. This is larger and proportional to the one in the base circuit. In order to explain the way in which this happens, the example of an n-p-n transistor is taken. The same principles are used for the p-n-p transistor except that the current carrier is holes rather than electrons and the voltages are reversed.

Operation of a bipolar junction transistor

The emitter in the n-p-n device is made of n-type material and here the majority carriers are electrons. When the base emitter junction is forward biased the electrons move from the n-type region towards the p-type region and the holes move towards the n-type region. When they reach each other they combine enabling a current to flow across the junction. When the junction is reverse biased the holes and electrons move away from one another resulting in a depletion region between the two areas and no current flows.

When a current flows between the base and emitter, electrons leave the emitter and flow into the base. Normally the electrons would combine when they reach this area. However the doping level in this region is very low and the base is also very thin. This means the most of the electrons are able to travel across this region without recombining with the holes. As a result the electrons migrate towards the collector, because they are attracted by the positive potential. In this way they are able to flow across what is effectively a reverse biased junction, and current flows in the collector circuit.

It is found that the collector current is significantly higher than the base current, and because the proportion of electrons combining with holes remains the same the collector current is always proportional to the base current. In other words varying the base current varies the collector current.

The ratio of the base to collector current is given the Greek symbol ?. Typically the ratio ? may be between 50 and 500 for a small signal transistor. This means that the collector current will be between 50 and 500 times that flowing in the base. For high power transistors the value of B is likely to be smaller, with figures of 20 not being unusual.

Summary of transistor junction bias scenarios

In looking at how a transistor works, the normal operation is to have the base emitter junction forward biased and the base collector junction reverse biased. Other scenarios are possible and the biasing arrangements are summarised below for both NPN and PNP variants.

NPN TRANSISTOR BIAS & OPERATION SUMMARY
ELECTRODE VOLTAGES	BASE EMITTER	COLLECTOR BASE	TRANSISTOR OPERATION
E < B < C	Forward	Reverse	Forward active. This is the normal mode for linear amplifiers.
E < B > C	Forward	Forward	Saturation, i.e. the transistor is switched hard on. This will occur in switching circuits
E > B < C	Reverse	Reverse	Cut-off. This occurs when the transistor is switched hard off and no collector current flows.
E > B > C	Reverse	Forward	Reverse active. This mode is not normally used as it effectively reverses the emitter and collector connections. Lower performance levels are achieved.