Transistors are the major element of modern electronics. They serve as amplifiers in audio and video equipment, and as electronic switches in computers. The basic principle is that a transistor can control its output current according to small changes in the input, much as a light switch can control hundreds of watts power at the flick of a finger.

A pnp transistor is constructed by sandwiching a thin layer of n-type semiconductor between two segments of p-type material.

The small battery on the left serves as the switch. Without that portion of the circuit, the large battery VC cannot pump current due to the reverse bias of the n-p junction in right-hand loop. By adding the small voltage VE, a current flows in the left-hand loop, which floods the narrow n-type region with charge. That charge allows a strong current to flow in the right hand loop. Thus a small change in the voltage VE creates a large effect in the output current IC.

As described, the transistor can serve as either a switch or an amplifier. The circuit diagram for the transistor is shown on the right. The wire with the arrow represents the emitter; the central wire (shown here going downward) is the base, and the remaining wire (shown here at the upper right) is the collector.

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