Transistors

Transistors are the crucial basic element in modern electronics. They serve as amplifiers in audio and video equipment, and as a electronic switches in computers. The basic principle is that a single transistor can control an output current according to small changes in the input, just 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 upper left lead represents the emitter, the upper right lead represents the collector and the lower lead is the base.

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