A transistor is the most crucial element in modern electronics. It serves as an amplifier and as a switch. A single transistor will adjust the output according to a small change in the input. Just as a light switch can supply many hundreds of watts power by the flick of a finger. Before transistors, amplification and switching was done with vacuum tubes which are enormously bulky and produce a great deal of heat. Since a single microprocessor may hold near 5 million transistors, the advantage to 5 million vacuum tubes is obvious.
A transistor is constructed by sandwiching a 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 can not pump current due to the n-p reverse bias of the right-hand loop. By adding a small voltage VE, a healthy current flows through the left-hand loop which floods the narrow n-type region with charge which then destroys the ability of the n-p junction to stop the current. Thus a small voltage change in VE creates a large effect. Thus it 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.