The transistor is an arrangement of semiconductor materials that share common physical boundaries. Materials most commonly used are silicon, gallium-arsenide, and germanium, into which impurities have been introduced by a process called "doping." In n -type semiconductors the impurities or dopants result in an excess of electrons, or negative charges; in p -type semiconductors the dopants lead to a deficiency of electrons and therefore an excess of positive charge carriers or "holes."The Junction Transistor
The n-p-n junction transistor consists of two n -type semiconductors (called the emitter and collector) separated by a thin layer of p -type semiconductor (called the base). The transistor action is such that if the electric potentials on the segments are properly determined, a small current between the base and emitter connections results in a large current between the emitter and collector connections, thus producing current amplification. Some circuits are designed to use the transistor as a switching device; current in the base-emitter junction creates a low-resistance path between the collector and emitter. The p-n-p junction transistor, consisting of a thin layer of n -type semiconductor lying between two p -type semiconductors, works in the same manner, except that all polarities are reversed.
A very important type of transistor developed after the junction transistor is the field-effect transistor (FET). It draws virtually no power from an input signal, overcoming a major disadvantage of the junction transistor. An n -channel FET consists of a bar (channel) of n -type semiconductor material that passes between and makes contact with two small regions of p -type material near its center. The terminals attached to the ends of the channel are called the source and the drain; those attached to the two p -type regions are called gates. A voltage applied to the gates is directed so that no current exists across the junctions between the p - and n -type materials; for this reason it is called a reverse voltage. Variations of the magnitude of the reverse voltage cause variations in the resistance of the channel, enabling the reverse voltage to control the current in the channel. A p -channel device works the same way but with all polarities reversed.
The metal-oxide semiconductor field-effect transistor (MOSFET) is a variant in which a single gate is separated from the channel by a layer of metal oxide, which acts as an insulator, or dielectric. The electric field of the gate extends through the dielectric and controls the resistance of the channel. In this device the input signal, which is applied to the gate, can increase the current through the channel as well as decrease it.