Question

A NPN transistor conducts when:
A. Both collector and emitter are positive with respect to the base
B. Collector is positive and the emitter is negative with respect to the base.
C. Collector is positive and emitter is at the same potential as the base
D. Both collector and emitter are negative with respect to the base

Answer 1

Hint: The emitter-base junction should be forward biased and the collector-base junction should be reverse biased for conduction by the NPN transistor. An n-type semiconductor has electrons as majority charge carriers while a p-type semiconductor has holes as majority charge carriers.

Complete step by step solution:
In an NPN transistor, the emitter and collector are n-type semiconductors and the base is a p-type semiconductor. In order for conduction to occur through the transistor, the emitter should be forward biased. This means that the emitter should be connected to the negative terminal of the battery with respect to the base.

As a result, electrons which are the majority charge carriers in n-type semiconductors will easily travel from n-type emitter to p-type base. This is called forward biasing. While crossing the p-type base, a few electrons and holes (present in the base) recombine resulting in base current.

Most of the electrons cross the base and reach the collector-base junction which is at positive potential compared to the base so the electrons move towards it and this collector current is known as the transistor output. In the case of the collector-base junction, the n-type collector is at positive potential compared to the base and hence this junction is called reverse biased. So, an NPN transistor will conduct only when the collector is positive and the emitter is negative with respect to the base.

Hence option (B) is the correct answer.

Note: The emitter is heavily doped to provide a large number of charge carriers for conduction. The base region has a very small thickness to avoid recombination of the electron-hole pair. The collector is wider than both the emitter and base to collect the charge carriers. All junctions are to be biased for conduction.