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What is a MOSFET Transistor?

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  • Written By: John Markley
  • Edited By: Melissa Wiley
  • Last Modified Date: 09 November 2016
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A MOSFET transistor is a semiconductor device that switches or amplifies signals in electronic devices. MOSFET is an acronym for metal–oxide–semiconductor field-effect transistor. The name can be variously written as MOSFET, MOS FET, or MOS-FET; the term MOSFET transistor is commonly used, despite its redundancy. The purpose of a MOSFET transistor is to affect the flow of electric charges through a device by using small amounts of electricity to influence the flow of much larger amounts. MOSFETs are the most commonly used transistors in modern electronics.

The MOSFET transistor is ubiquitous in modern life because it is the transistor type most commonly used in integrated circuits, the basis of almost all modern computers and electronic devices. The MOSFET transistor is well-suited for this role due to its low power consumption and dissipation, low waste heat, and low mass production costs. A modern integrated circuit can contain billions of MOSFETs. MOSFET transistors are present in devices ranging from cellular phones and digital watches to enormous supercomputers used for complex scientific calculations in fields such as climatology, astronomy, and particle physics.

A MOSFET has four semiconductor terminals, called the source, gate, drain, and body. The source and drain are located in the body of the transistor, while the gate is above these three terminals, positioned between the source and drain. The gate is separated from the other terminals by a thin layer of insulation.

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A MOSFET can be designed to use either negatively charged electrons or positively charged electron holes as electric charge carriers. The source, gate, and drain terminals are designed to have an excess of either electrons or electron holes, giving each a negative or positive polarity. The source and drain are always the same polarity, and the gate is always the opposite polarity of the source and drain.

When the voltage between the body and gate is increased and the gate receives an electric charge, electric charge carriers of the same charge are repelled from the area of the gate, creating what is called a depletion region. If this region becomes large enough, it will create what is called an inversion layer at the interface of the insulating and semiconducting layers, providing a channel where charge carriers of the opposite polarity of the gate can flow easily. This allows large amounts of electricity to flow from the source to the drain. Like all field-effect transistors, each individual MOSFET transistor uses either positive or negative charge carriers exclusively.

MOSFET transistors are made primarily of silicon or a silicon-germanium alloy. The properties of the semiconductor terminals can be altered by adding small impurities of substances such as boron, phosphorous, or arsenic, a process called doping. The gate is usually made of polycrystalline silicon, though some MOSFETs have gates made of polysilicon alloyed with metals such as titanium, tungsten, or nickel. Extremely small transistors use gates made from metals such tungsten, tantalum, or titanium nitride. The insulating layer is most commonly made of silicon dioxide (SO2), though other oxide compounds are also used.

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