NPN is one of the two types of bipolar transistors, the second being PNP. The letters "N" and "P" refer to the majority charge carriers inside the different regions of the transistor. Most transistors used today are NPN, since this is the easiest and most cost effective to make from silicon.
NPN transistors consist of a layer of P-doped (the doping agent is often boron) semiconductor (the "base") between two N-doped (often made with arsenic) layers. NPN transistors are commonly operated with the emitter at ground and the collector at a positive voltage.
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