One of the most common uses for transistors in an electronic circuit is simple switches. In short, a transistor drives the current across the collector-emitter trajectory only when a voltage is applied to the base. When there is no basic voltage, the switch is off. When there is basic voltage, the switch is on.
The essential utility of a transistor comes from its ability to use a small signal applied between a pair of terminals to control a much larger signal at another pair of terminals. This property is called gain. It can produce a stronger output signal, a voltage or current, proportional to a lower input signal; ie it can function as an amplifier. Alternatively, the transistor can be used to start or stop the current in a circuit as an electrically controlled switch, where the amount of current is determined by other circuit elements.
There are two types of transistors, which have small differences in the way they are used in a circuit. A bipolar transistor has terminals labeled with base, collector and transmitter. A small current at the base terminal (which circulates between the base and the transmitter) can control or switch a much larger current between the collector terminals and the emitter. For a field effect transistor, the terminals are labeled as gate, source and drain, and a gate voltage can control a current between source and drain.
The image represents a typical bipolar transistor in a circuit. Depending on the base current, charging between the transmitter terminals and the collector will be charged. Because the base connections and the internal transmitter behave like a semiconductor diode, there is a voltage drop between the base and the transmitter while the base current exists. The amount of this voltage depends on the material on which the transistor is made and is called VBE.