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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; curn 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.
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