An npn transistor is the more common flavour of BJT: an n-type emitter, a thin p-type base, and an n-type collector, in that order. It is the device used for every worked example in this course because the polarity-reversed PNP transistor just mirrors it.
The emitter is heavily-doped n-type, so it has a huge surplus of mobile electrons. The base is thin and lightly-doped p-type. The collector is n-type. In active mode the emitter–base junction (EBJ) is forward-biased and the collector–base junction (CBJ) is reverse-biased. Forward-biasing the EBJ launches a flood of electrons from the emitter into the base. Because the base is thin and lightly doped, most of those electrons diffuse straight across it without recombining with the base’s holes. As soon as they reach the reverse-biased CBJ, its strong electric field sweeps them across and out the collector terminal — this is Minority-carrier injection in action.
So the conventional current directions in an npn: collector current flows into the collector, base current flows into the base, and emitter current flows out of the emitter (electrons enter through the emitter, so conventional current leaves it). KCL at the device gives
and in active mode , where is the Common-emitter current gain and is the Common-base current gain.
The npn is preferred over the pnp because its working carriers are electrons, and electrons have higher mobility than the holes that carry current in a pnp. Higher carrier mobility means faster transit across the base, so an npn is faster than a pnp of the same geometry. That is the same reason an n-channel MOSFET is faster than a p-channel one.
Its circuit symbol has the emitter arrow pointing out of the transistor — the mnemonic is “NPN, Not Pointing iN” — and that arrow direction is also the direction of conventional emitter current in active mode.