A transistor can be understood as a combination of two diodes, which creates two p-n junctions. Depending on the doping sequence, these junctions form either an npn or a pnp transistor.
The middle region is called the base (B), while the two outer terminals are the emitter (E) and the collector (C).
- In an npn transistor, the base is lightly p-doped ($p^+$),
- The collector is lightly n-doped ($n^+$),
- And the emitter is heavily n-doped ($n^{++}$).
First, a voltage is applied between the collector and emitter:
- The positive pole of the power source is connected to the collector,
- The negative pole to the emitter.
If a voltage is then applied between base and emitter in forward bias, the p-n junction at the base becomes conductive, and negative charge carriers (electrons) flood into the base.
Because the base region is very thin and weakly doped, most of these electrons cannot recombine there. Instead, they pass through the second p-n junction into the collector. This makes the collector–emitter connection conductive, so a continuous current flows as long as there is a base current. If the base current is interrupted, the collector–emitter current also immediately stops.
There are three basic circuit configurations for transistors:
- Common base
- Common emitter
- Common collector
The most widely used is the common emitter configuration (example for an npn transistor):
- When a forward bias voltage $U_1$ is applied between base and emitter, a base current flows.
- If another voltage $U_2$ is applied between collector and emitter, a collector current begins to flow as well.
- This collector current can be much larger than the base current.
- The output voltage $U_3$ is taken from the collector.
Since the collector current is largely proportional to the base current, transistors are used as amplifiers.
Transistors can also act as electronic switches, for example in light barriers, replacing earlier mechanical relays.