Part 1: NPN Transistors

The NPN transistor is formed by sandwiching a layer of P-type semiconductor between two layers of N-type semiconductor.

NPN Transistor

The collector and emitter of the transistor can be connected in series with a power source and a load, much as a switch would be in a simple electrical circuit. In this case the switch is operated by a small current flowing from the emitter to the base (the base current). No current will flow from the emitter to the collector until a small voltage (at least 0.7 volts) is applied to the base of the transistor.

NPN Transistor on

The current from the emitter to the collector (collector current) is much larger than the base current. The collector current is also proportional to the base current: that is, as the base current increases, the collector current increases, until the transistor reaches is maximum capability.

NPN Transistor Symbol

Note that the emitter is signified by an arrow on the symbol.