Transistors

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 V_C cannot pump current due to the reverse bias of the n-p junction in right-hand loop. By adding the small voltage V_E, 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 V_E creates a large effect in the output current I_C.

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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