With neat diagram explain working of NPN transistor.
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A transistor is formed of two p-n junctions. For unbiased p-n junctions, the depletion regions are formed. The depletion regions formed at the B-E and C-B junctions of n-p-n transistor. 

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Working Principle: When the n-p-n transistor is biased, as shown in fig.(b), such that the emitter-base junction is forward biased and collector-base junction is reverse biased, the minority and majority carriers are set into the motion. The majority carriers electrons from n-region enters into p-region and holes from p-region enters into n region. Since the base is lightly doped than the emitter, almost all the current flowing across the B-E junction consists of electrons entering the base from the emitter. Hence the electrons are the majority carriers in n-p-n transistor. Some of the electrons entering into the base region do not reach the collector region. Instead they flow out of the base terminal via the base connection as shown in fig.(c) due to recombination. As the base region is very thin and lightly doped, there are very few holes available in the base region for recombination. Hence about 2% electrons will flow out of base due to recombination. The remaining 98% electrons cross the reverse biased collector-base junction to constitute the collector current. They cross the collector region and collected by the supply Vcc. The emitter current is thus equal to the sum of the base current and collector current. = + c

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N-p-n transistor is made by sandwiching thin layer of p-type semiconductor between two layers of n-type semiconductor. It has three terminals - Emitter, Base and collector. The npn transistor has two supplies, one is connected through the emitter base and one through the collector base. The supply is connected such that emitter-base are forward biased and collector base are reverse biased. It means, Base has to be more positive than the emitter and in turn, the collector must be more positive than the base. The current flow in this type of transistor is carried through movement of electrons. Emitter emits electrons which are pulled by the base as it is more positive. This end up in the collector as it is more positive. In this way, current flows in the transistor.

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