Transistors

Transistors are electronic devices made of semiconductor material that amplify a signal or open or close a circuit. Invented in 1947 at Bell Labs, transistors have become the key ingredient of all digital circuits, including computers. It is the solid-state analog to the triode electron tube; the transistor has replaced the electron tube for virtually all-common applications. 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 a process called “doping” has introduced impurities. 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. 

Junction bipolar transistors (BJT) have two main functional designs:  n-p-n and p-n-p junction transistors. n-p-n transistors consist 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. p-n-p junction transistors consist 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. 

Field-effect transistors were developed after the junction transistor is the field-effect transistor (FET). They draw 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.

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.

• Transistor-Transistor Logic (TTL)
  Transistor-transistor logic (TTL) is a class of digital circuits built from bipolar junction transistors (BJT), diodes and resistors. It is notable, as it was the base for the first widespread...
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• Bipolar Junction Transistors
  Bipolar junction transistors (BJT) are used in general-purpose applications. They consist of two sections of one type of semiconductor (N or P) around a middle slab of the other type. The junctions...
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• High Electron Mobility Transistors (HEMT)
  High electron mobility transistors (HEMTs) are used in microwave circuit applications. These transistors behave much like conventional field effect transistors (FETs): a conducting channel between...
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Part Numbers for Transistors