![]() Since the base is lightly doped and very thin, hence only a few holes (less than 5%) combine with the electrons and constitute base current I B. The forward bias causes the holes in the p-type emitter to flow towards the base, thus constituting the emitter current I E.Īs the holes cross the emitter and enter into the n-type base, they tend to combine with the electrons. The figure below, shows a pnp transistor with emitter-base junction forward biased by voltage V EB and base-collector junction reverse biased by voltage V CB. Now it is clear that the emitter current is the sum of base current and collector current. The remaining electrons (more than 95%) cross over the base into the collector region and constitute collector current I C. As we already know, the base is lightly doped and very thin, hence, a very small amount of holes are available to be combined with the electrons (nearly 5%) from emitter and constitute the base current I B. Now as these electrons from emitter flow through the p-type base, they combine with the holes. This constitutes the emitter current I E . The forward bias causes the electrons in the n-type emitter to flow towards the base. The figure below shows an npn transistor with its emitter-base junction forward biased by voltage V EB and base-collector junction reverse biased by voltage V CB. Since the resistance of the emitter-base junction is very small as compared to the base-collector junction resistance, therefore the forward bias applied to the emitter-base junction is very small as compare to the reverse bias applied to the base-collector junction NPN Transistor Working The emitter-base junction is always forward biased where as the base-collector junction is always reverse biased. The base is lightly doped so that it can pass most of the charge carriers that arrives from the base to the collector. The emitter is heavily doped so that a large number of charge carriers become available to be injected into the base. In a transistor, the base is much thinner than the emitter, however, the collector is wider than both base and emitter. The base-emitter junction is forward biased, thus allows a low resistance path for the emitter circuit.Whereas, the base-collector junction is reverse biased and provides a high resistance path in the collector circuit. The middle section which forms two pn junctions between the emitter and collector is called as base. Similarly, the collector (p-type) of the p-n-p transistor has reverse bias and receives holes. The collector (n-type) of the n-p-n transistor has reverse bias and receives electrons. The collector is always reverse biased with respect to base so that it can remove charges from its junction with the base. The section on the other side which collects the charge carriers is known as collector. Similarly, in the figure below, the emitter (p-type) of the p-n-p transistor is forward biased and supplies holes to its junction with the base. In the figure below, the emitter (n-type) of the n-p-n transistor is forward biased and supplies electrons to its junction with the base. It is always forward biased with respect to the base in order to supply a large number of majority carriers. The section on one side which supplies charge carriers (electrons or holes) is called as emitter. The middle section is called the base and it forms two junctions between the emitter and collector. The section on one side is called emitter and the section on the other side is knows as collector.
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