Editing Transistor (section)

===Field-effect transistor (FET)===
{{Main|Field-effect transistor}}
{{See also|JFET}}
[[File:Threshold formation nowatermark.gif|thumb|right|Operation of an [[FET]] and its {{mvar|I<sub>d</sub>}}-{{mvar|V<sub>g</sub>}} curve. At first, when no gate voltage is applied, there are no inversion electrons in the channel, so the device is turned off. As gate voltage increases, the inversion electron density in the channel increases, the current increases, and the device turns on.]]
The ''[[field-effect transistor]]'', sometimes called a ''unipolar transistor'', uses either electrons (in ''n-channel FET'') or holes (in ''p-channel FET'') for conduction. The four terminals of the FET are named ''source'', ''gate'', ''drain'', and ''body'' (''substrate''). On most FETs, the body is connected to the source inside the package, and this will be assumed for the following description.

In a FET, the drain-to-source current flows via a conducting channel that connects the ''source'' region to the ''drain'' region. The conductivity is varied by the electric field that is produced when a voltage is applied between the gate and source terminals, hence the current flowing between the drain and source is controlled by the voltage applied between the gate and source. As the gate–source voltage ({{mvar|V<sub>GS</sub>}}) is increased, the drain–source current ({{mvar|I<sub>DS</sub>}}) increases exponentially for {{mvar|V<sub>GS</sub>}} below threshold, and then at a roughly quadratic rate:  ({{math|''I<sub>DS</sub>'' ∝  (''V<sub>GS</sub>'' − ''V<sub>T</sub>'')<sup>2</sup>}}, where {{mvar|V<sub>T</sub>}} is the threshold voltage at which drain current begins)<ref name=horowitz-hill>{{cite book|last=Horowitz|first=Paul|author-link=Paul Horowitz|author2=Winfield Hill |title=The Art of Electronics|edition=2nd|year=1989|publisher=Cambridge University Press|isbn=978-0-521-37095-0|page=[115]|title-link=The Art of Electronics|author2-link=Winfield Hill}}</ref> in the [[space charge|space-charge-limited]] region above threshold. A quadratic behavior is not observed in modern devices, for example, at the [[65 nanometer|65 nm]] technology node.<ref name=Sansen>
{{cite book|author=Sansen, W. M. C. |title=Analog design essentials|year= 2006|page=§0152, p. 28|publisher=Springer|location=New York, Berlin|isbn=978-0-387-25746-4}}</ref>

For low noise at narrow [[bandwidth (signal processing)|bandwidth]], the higher input resistance of the FET is advantageous.

FETs are divided into two families: ''junction FET'' ([[JFET]]) and ''insulated gate FET'' (IGFET). The IGFET is more commonly known as a ''metal–oxide–semiconductor FET'' ([[MOSFET]]), reflecting its original construction from layers of metal (the gate), oxide (the insulation), and semiconductor. Unlike IGFETs, the JFET gate forms a [[p–n diode]] with the channel which lies between the source and drains. Functionally, this makes the n-channel JFET the solid-state equivalent of the vacuum tube [[triode]] which, similarly, forms a diode between its [[Control grid|grid]] and [[cathode]]. Also, both devices operate in the ''depletion-mode'', they both have a high input impedance, and they both conduct current under the control of an input voltage.

Metal–semiconductor FETs ([[MESFET]]s) are JFETs in which the [[Reverse-biased|reverse biased]] p–n junction is replaced by a [[metal–semiconductor junction]]. These, and the HEMTs (high-electron-mobility transistors, or HFETs), in which a two-dimensional electron gas with very high carrier mobility is used for charge transport, are especially suitable for use at very high frequencies (several GHz).

FETs are further divided into ''depletion-mode'' and ''enhancement-mode'' types, depending on whether the channel is turned on or off with zero gate-to-source voltage. For enhancement mode, the channel is off at zero bias, and a gate potential can ''enhance'' the conduction. For the depletion mode, the channel is on at zero bias, and a gate potential (of the opposite polarity) can ''deplete'' the channel, reducing conduction. For either mode, a more positive gate voltage corresponds to a higher current for n-channel devices and a lower current for p-channel devices. Nearly all JFETs are depletion-mode because the diode junctions would forward bias and conduct if they were enhancement-mode devices, while most IGFETs are enhancement-mode types.

====Metal–oxide–semiconductor FET (MOSFET)====
{{Main|MOSFET}}
The metal–oxide–semiconductor field-effect transistor ([[MOSFET]], MOS-FET, or MOS FET), also known as the metal–oxide–silicon transistor (MOS transistor, or MOS),<ref name="computer history-transistor"/> is a type of field-effect transistor that is [[Semiconductor device fabrication|fabricated]] by the [[thermal oxidation|controlled oxidation]] of a semiconductor, typically [[silicon]]. It has an insulated [[Metal gate|gate]], whose voltage determines the conductivity of the device. This ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic [[signal (electrical engineering)|signals]]. The MOSFET is by far the most common transistor, and the basic building block of most modern [[electronics]].<ref name="triumph"/> The MOSFET accounts for 99.9% of all transistors in the world.<ref name="computerhistory2018">{{cite web |title=13 Sextillion & Counting: The Long & Winding Road to the Most Frequently Manufactured Human Artifact in History |url=https://www.computerhistory.org/atchm/13-sextillion-counting-the-long-winding-road-to-the-most-frequently-manufactured-human-artifact-in-history/ |date=April 2, 2018 |website=[[Computer History Museum]] |access-date=July 28, 2019}}</ref>