The net result is that a large number of electrons that are injected into the emitter, are "swept across" the base by the positive voltage of This is because of the very low density of holes in the base region and the fact that it is extremely thin. Number of the electrons from the emitter, are swept across the base by the positive voltage of the collector, before they get chance to encounterĪnd then recombine with the holes. This consist of holes flowing in the base and electrons flowing in the emitter, which recombine at the junction. The base current still flows due to the forward biased baseĮmitter junction. It can be seen that the applied polarity causes the base collector junction to be reversed biased. Is limited by the lower doping of the base region. Therefore the current that flows in the base (Ib), (and across the base emitter junction ) Much more heavily doped than the base region. If an external voltage is applied to forward bias the base emitter junction, then as expected current will flow across it.
In fact the difference in dimensions along with the difference in doping levels between each region, are critical to theįorward biased of the base emitter junction. It is very important to note that the diagram does not reflect the actual comparative dimensions of each region/layer The three different regions of the transistor are called theĮmitter, base and collector. The figure below shows a diagram for an NPN transistor along with the circuit symbol. The term bipolar, refers to the fact that in this type of transistor the current flow involves two types of majority charge carriers, (i.e. (The PNP has the same principle of operation, except that the role of the charge carriers is interchanged and the applied voltages have opposite In this section we will describe the operating principles of an NPN transistor. (A PNP transistor has the opposite arrangement).
NPN (and PNP) transistors.Īn NPN transistor can crudely be described, as being made up of a layer of P type semiconductor, "sandwiched" between two pieces of
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