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solid-state device for amplifying, controlling, and generating electrical signals. Transistors are used in a wide array of electronic equipment, ranging from pocket calculators and radios to industrial robots and communications satellites. 

The transistor was invented in 1947 by three American physicists at the Bell Telephone Laboratories, John Bardeen, Walter H. Brattain, and William B. Shockley. It proved to be a viable alternative to the vacuum tube, and by the late 1950s supplanted the latter in many applications. Transistors played a pivotal role in the advancement of electronics, their small size, low heat generation, high reliability, and relatively small power requirements making possible the miniaturization of complex circuitry such as required by computers. During the late 1960s and '70s individual transistors were superseded by integrated circuits in which a multitude of transistors and other components (e.g., diodes and resistors) are formed on a single tiny wafer of semiconducting material. See also integrated circuit.

Transistors are made up of layers of different semiconductors produced by the addition of certain impurities (e.g., arsenic or boron) to silicon. These impurities affect the way electric current moves through the silicon. In semiconductors known as n-type, the primary electric charge carriers are free electrons; in those called p-type, the principal charge carriers are positively charged holes.(That is to say, when an atom such as boron that has only three outer electrons is substituted for a silicon atom with four electrons, a vacancy, or hole, is created in the valence band.) Holes in semiconductors move about as readily as electrons do, but their motion is in a direction opposite to that of electrons since they are positively charged. 



There are two general types of transistors: (1) the bipolar junction transistor (BJT), and (2) the field-effect transistor (FET). The BJT, composed of two closely coupled p-n junctions, is bipolar in that both electrons and holes are involved in the conduction process. It is readily able to deliver a change in output voltage in response to a change in input current. This type of transistor is widely used as an amplifier and is also a key component in oscillators, high-speed integrated circuits, and switching circuits. 

In contrast to the BJT, the FET is a unipolar device—i.e., its conducting process primarily involves only one kind of charge carrier. It can be built either as a metal-oxide-semiconductor field-effect transistor (MOSFET) or as a junction field-effect transistor (JFET). 
Since the mid-1980s the MOSFET has surpassed the BJT in importance and become the predominant component of very-large-scale integrated (VLSI) circuits. Not only does it consume much less power than the BJT, but it can be scaled down to smaller dimensions than the latter with greater ease. Other commercially important types of FETs are the metal-semiconductor field-effect transistor (MESFET) and the closely related JFET. The MESFET can be employed in both analog and digital circuits, and is particularly useful for microwave amplification.