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Thursday, 17 November 2011

Biasing and Working of PNP Transistor

For the normal operation of a transistor, it is necessary to provide voltage of right polarities to both of its junctions.
For the accurate and normal function of a transistor, its emitter-base junction should be forward biased and collector-base junction should be always reverse biased. As shown in the figure.

In the given method of biasing of PNP transistor, the emitter-base junction is forward biased, while collector-base junction is reverse biased.
Due to forward biasing of emitter-base junction, the majority carriers (holes) flow from emitter towards base. Thus, a big amount of holes crosses the junction and enters base. As the holes enter the base, they combine with electrons present there, (This process known as recombination).Since the base is lightly doped, so the recombination of holes and electrons in base is very low, (Almost 2% to 5%). Thus 95 to 98 percent of holes that came from emitter could not recombine with electrons.

Since the collector-base junction is reverse biased, so holes cannot flow from collector to base due to high resistance. But holes start to flow in the opposite direction (i.e. base to collector) because collector voltage attracts almost 95% holes present in base region. Thus the holes provided by emitter, travelling through base, go to collector-base junction and conventional current starts to flow in PNP transistor.
This should be kept in mind that collector current and emitter current is almost equal but base current is very low. 




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