View source for JFET

{{refimprove|date=September 2015}}
{{short description|Type of field-effect transistor}}
{{Infobox electronic component
|name              = JFET
|image             = JFET cross section.svg
|image_size        = 270px
|caption           =[[Electric current]] from ''source'' to ''drain'' in a '''p-channel JFET''' is restricted when a [[voltage]] is applied to the ''gate''.
|type              = Active
|working_principle = 
|invented          = 
|first_produced    =
|symbol            = [[Image:IEEE 315-1975 (1993) 8.6.10.1.b.svg|100px]] [[Image:IEEE 315-1975 (1993) 8.6.11.1.b.svg|100px]]
|pins              =  drain, gate, source
}}

The '''junction field-effect transistor''' ('''JFET''') is one of the simplest types of [[field-effect transistor]].<ref>{{cite web |url=http://www.linearsystems.com/lsdata/others/LIS_White_Paper_Consider_Discrete_JFET.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.linearsystems.com/lsdata/others/LIS_White_Paper_Consider_Discrete_JFET.pdf |archive-date=2022-10-09 |url-status=live |title=Discrete JFET |last=Hall |first=John |website=linearsystems.com}}</ref> JFETs are three-terminal [[semiconductor]] devices that can be used as [[electronics|electronically]] controlled [[switch]]es or [[Voltage-controlled resistor|resistors]], or to build [[Amplifier|amplifiers]]. 

Unlike [[bipolar junction transistors]], JFETs are exclusively [[voltage]]-controlled in that they do not need a [[biasing|biasing current]]. [[Electric charge]] flows through a [[semiconducting]] channel between ''source'' and ''drain'' [[Terminal (electronics)|terminals]]. By applying a reverse bias [[voltage]] to a ''gate'' terminal, the channel is ''[[Channel length modulation|pinched]]'', so that the [[electric current]] is impeded or switched off completely. A JFET is usually conducting when there is zero voltage between its gate and source terminals. If a potential difference of the proper [[Electrical polarity|polarity]] is applied between its gate and source terminals, the JFET will be more [[resistive]] to current flow, which means less current would flow in the channel between the source and drain terminals.

JFETs are sometimes referred to as [[depletion-mode]] devices, as they rely on the principle of a [[depletion region]], which is devoid of majority [[Charge carrier|charge carriers]]. The depletion region has to be closed to enable current to flow.

JFETs can have an [[n-type semiconductor|n-type]] or [[p-type semiconductor|p-type]] channel. In the n-type, if the voltage applied to the gate is negative with respect to the source, the current will be reduced (similarly in the p-type, if the voltage applied to the gate is positive with respect to the source). Because a JFET in a [[common source]] or [[common drain]] configuration has a large [[input impedance]]<ref>{{Cite web |title=Junction Field Effect Transistor |url=https://www.electronics-tutorials.ws/transistor/tran_5.html |url-status=live |archive-url=https://web.archive.org/web/20220131124209/https://www.electronics-tutorials.ws/transistor/tran_5.html |archive-date=2022-01-31 |access-date=2022-06-19 |website=Electronics Tutorials}}</ref> (sometimes on the order of 10<sup>10</sup>&nbsp;[[Ohm|ohms]]), little current is drawn from circuits used as input to the gate.

== History ==
A succession of FET-like devices was patented by [[Julius Edgar Lilienfeld|Julius Lilienfeld]] in the 1920s and 1930s. However, [[materials science]] and fabrication technology would require decades of advances before FETs could actually be manufactured.

JFET was first patented by [[Heinrich Welker]] in 1945.<ref>{{cite book |title=The Physics of Semiconductors|author=Grundmann, Marius|isbn=978-3-642-13884-3 |publisher=Springer-Verlag|year=2010}}</ref> During the 1940s, researchers [[John Bardeen]], [[Walter Houser Brattain]], and [[William Shockley]] were trying to build a FET, but failed in their repeated attempts. They discovered the [[point-contact transistor]] in the course of trying to diagnose the reasons for their failures. Following Shockley's theoretical treatment on JFET in 1952, a working practical JFET was made in 1953 by [[George C. Dacey]] and [[Ian Munro Ross|Ian M. Ross]].<ref name="sit"/> Japanese engineers [[Jun-ichi Nishizawa]] and Y. Watanabe applied for a patent for a similar device in 1950 termed [[static induction transistor]] (SIT). The SIT is a type of JFET with a short channel.<ref name="sit">[https://link.springer.com/chapter/10.1007%2F978-1-4684-7263-9_11#page-1 Junction Field-Effect Devices], ''Semiconductor Devices for Power Conditioning'', 1982.</ref>

High-speed, high-voltage switching with JFETs became technically feasible following the commercial introduction of [[Silicon carbide#Power electronic devices|Silicon carbide]] (SiC) [[Wide-bandgap semiconductor|wide-bandgap]] devices in 2008. Due to early difficulties in manufacturing &mdash; in particular, inconsistencies and low yield &mdash; SiC JFETs remained a niche product at first, with correspondingly high costs. By 2018, these manufacturing issues had been mostly resolved. By then, SiC JFETs were also commonly used in conjunction with conventional low-voltage Silicon MOSFETs.<ref name="Flaherty2018">{{citation|last=Flaherty|first=Nick|date=October 18, 2018|title=Third generation SiC JFET adds 1200 V and 650 V options|periodical=EeNews Power Management|url=https://www.eenewspower.com/news/third-generation-sic-jfet-adds-1200-v-and-650-v-options}}.</ref> In this combination, SiC JFET + Si MOSFET devices have the advantages of wide band-gap devices as well as the easy gate drive of MOSFETs.<ref name="Flaherty2018" />

== Structure ==
The JFET is a long channel of [[semiconductor]] material, [[doping (semiconductor)|doped]] to contain an abundance of positive [[electric charge|charge]] carriers or [[Electron hole|holes]] (''p-type''), or of negative carriers or [[electron]]s (''n-type''). [[Ohmic contact]]s at each end form the source (S) and the drain (D). A [[P-N junction|pn-junction]] is formed on one or both sides of the channel, or surrounding it using a region with doping opposite to that of the channel, and biased using an ohmic gate contact (G).

== Functions ==
[[Image:JFET n-channel en.svg|thumb|280px|I–V characteristics and output plot of an n-channel JFET]]

JFET operation can be compared to that of a [[garden hose]]. The flow of water through a hose can be controlled by squeezing it to reduce the [[cross section (geometry)|cross section]] and the flow of [[electric charge]] through a JFET is controlled by constricting the current-carrying channel. The current also depends on the electric field between source and drain (analogous to the difference in [[Fluid pressure|pressure]] on either end of the hose). This current dependency is not supported by the characteristics shown in the diagram above a certain applied voltage. This is the ''saturation region'', and the JFET is normally operated in this constant-current region where device current is virtually unaffected by drain-source voltage. The JFET shares this constant-current characteristic with junction transistors and with thermionic tube (valve) tetrodes and pentodes.

Constriction of the conducting channel is accomplished using the [[Field effect (semiconductor)|field effect]]: a voltage between the gate and the source is applied to reverse bias the gate-source pn-junction, thereby widening the [[depletion layer]] of this junction (see top figure), encroaching upon the conducting channel and restricting its cross-sectional area. The depletion layer is so-called because it is depleted of mobile carriers and so is electrically non-conducting for practical purposes.<ref name=JFET> For a discussion of JFET structure and operation, see for example {{cite book |title=Electronics (fundamentals and applications) |chapter-url=https://books.google.com/books?id=n0rf9_2ckeYC&pg=PA269 |chapter=§13.2 Junction field-effect transistor (JFET) |author=D. Chattopadhyay |isbn=978-8122417807 |publisher=New Age International |pages=269 ''ff'' |year=2006}}</ref>

When the depletion layer spans the width of the conduction channel, ''pinch-off'' is achieved and drain-to-source conduction stops. Pinch-off occurs at a particular reverse bias (''V''<sub>GS</sub>) of the gate–source junction. The ''pinch-off voltage'' (V<sub>p</sub>) (also known as ''[[threshold voltage]]''<ref name=":2">{{cite web |title=Junction Field Effect Transistor (JFET) |url=https://coefs.uncc.edu/dlsharer/files/2012/04/J3a.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://coefs.uncc.edu/dlsharer/files/2012/04/J3a.pdf |archive-date=2022-10-09 |url-status=live |website=ETEE3212 Lecture Notes|quote=value of ''v''<sub>GS</sub> ... for which the channel is completely depleted  ... is called the '''threshold''', or '''pinch-off''', voltage and occurs at ''v''<sub>GS</sub> = ''V''<sub>GS(OFF)</sub>. ... This linear region of operation is called '''ohmic''' (or sometimes triode) ... Beyond the knee of the ohmic region, the curves become essentially flat in the '''active''' (or '''saturation''') '''region''' of operation.}}</ref><ref>{{Cite book |last1=Sedra |first1=Adel S. |title=Microelectronic Circuits |last2=Smith |first2=Kenneth C. |chapter=5.11 THE JUNCTION FIELD-EFFECT TRANSISTOR (JFET) |quote=At this value of ''v''<sub>GS</sub> the channel is completely depleted ... For JFETs the threshold voltage is called the '''pinch-off voltage''' and is denoted ''V''<sub>P</sub>. |chapter-url=https://global.oup.com/us/companion.websites/fdscontent/uscompanion/us/static/companion.websites/9780199339136/pdf/bonustopics.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://global.oup.com/us/companion.websites/fdscontent/uscompanion/us/static/companion.websites/9780199339136/pdf/bonustopics.pdf |archive-date=2022-10-09 |url-status=live}}</ref> or ''cut-off voltage''<ref>{{Cite book |last1=Horowitz |first1=Paul |url=https://www.worldcat.org/oclc/19125711 |title=The art of electronics |last2=Hill |first2=Winfield |date=1989 |publisher=Cambridge University Press |isbn=0-521-37095-7 |edition=2nd |location=Cambridge [England] |page=120 |oclc=19125711 |quote=For JFETs the gate-source voltage at which drain current approaches zero is called the "gate-source cutoff voltage", ''V''<sub>GS(OFF)</sub>, or the "pinch-off voltage", ''V''<sub>P</sub> ... For enhancement-mode MOSFETs the analogous quantity is the "threshold voltage"}}</ref><ref name=":0">{{Cite book |last1=Mehta |first1=V. K. |url=https://www.worldcat.org/oclc/741256429 |title=Principles of electronics |last2=Mehta |first2=Rohit |date=2008 |publisher=S. Chand |isbn=978-8121924504 |edition=11th |pages=513–514 |chapter=19 Field Effect Transistors |oclc=741256429 |quote='''Pinch off Voltage (''V''<sub>P</sub>).''' It is the minimum drain–source voltage at which the drain current essentially becomes constant. ... '''Gate–source cut off voltage ''V''<sub>GS (off)</sub>.''' It is the gate–source voltage where the channel is completely cut off and the drain current becomes zero. |chapter-url=http://www.talkingelectronics.com/Download%20eBooks/Principles%20of%20electronics/CH-19.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.talkingelectronics.com/Download%20eBooks/Principles%20of%20electronics/CH-19.pdf |archive-date=2022-10-09 |url-status=live}}</ref><ref name=":1">{{Cite book |last1=U. A. Bakshi |url=https://books.google.com/books?id=CLqqbq2ypZQC |title=Electronics Engineering |last2=Atul P. Godse |date=2008 |publisher=Technical Publications |isbn=978-81-8431-503-5 |page=10 |language=en |quote=Do not confuse cutoff with pinch off. The '''pinch-off voltage''' ''V''<sub>P</sub> is the value of the ''V''<sub>DS</sub> at which the drain current reaches a constant value for a given value of ''V''<sub>GS</sub>. ... The cutoff voltage ''V''<sub>GS(off)</sub> is the value of ''V''<sub>GS</sub> at which the drain current is 0.}}</ref>) varies considerably, even among devices of the same type. For example, ''V''<sub>GS(off)</sub> for the Temic J202 device varies from {{nowrap|−0.8 V}} to {{nowrap|−4 V}},<ref>{{cite web |url=http://docs-europe.origin.electrocomponents.com/webdocs/0027/0900766b80027bd1.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://docs-europe.origin.electrocomponents.com/webdocs/0027/0900766b80027bd1.pdf |archive-date=2022-10-09 |url-status=live |title=J201 data sheet |access-date=2021-01-22}}</ref> while the ''V''<sub>GS(off)</sub> for the J308 varies between {{nowrap|−1 V}} and {{nowrap|−6.5 V}}.<ref>{{cite web |url=https://www.linearsystems.com/_files/ugd/7e8069_1d25831e77cf4fa6acf32aa92dff50a2.pdf |title=U/J/SST308 series—Single N-Channel High Frequency JFET Amplifier|date=2019-07-25|access-date=2025-02-03}}</ref> (Confusingly, the term ''pinch-off voltage'' is also used to refer to the ''V''<sub>DS</sub> value that separates the linear and saturation regions.<ref name=":0" /><ref name=":1" />)

To switch off an '''n'''-channel device requires a '''n'''egative gate–source voltage (''V''<sub>GS</sub>). Conversely, to switch off a '''p'''-channel device requires '''p'''ositive ''V''<sub>GS</sub>.

In normal operation, the electric field developed by the gate blocks source–drain conduction to some extent.

Some JFET devices are symmetrical with respect to the source and drain.

== Schematic symbols ==
[[Image:JFET N-dep symbol.svg|thumb|100px|[[electronic symbol|Circuit symbol]] for an n-channel JFET]]
[[Image:JFET P-dep symbol.svg|thumb|100px|Circuit symbol for a p-channel JFET]]

The JFET gate is sometimes drawn in the middle of the channel (instead of at the drain or source electrode as in these examples). This symmetry suggests that "drain" and "source" are interchangeable, so the symbol should be used only for those JFETs where they are indeed interchangeable.

The symbol may be drawn inside a circle (representing the envelope of a discrete device) if the enclosure is important to circuit function, such as dual matched components in the same package.<ref>{{Cite book |url=https://www.julesbartow.com/Construction/ANSI%20Y32.2-1975.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.julesbartow.com/Construction/ANSI%20Y32.2-1975.pdf |archive-date=2022-10-09 |url-status=live |title=ANSI Y32.2-1975 |chapter=A4.11 Envelope or Enclosure |quote=The envelope or enclosure symbol may be omitted from a symbol referencing this paragraph, where confusion would not result}}</ref>

In every case the arrow head shows the polarity of the P–N junction formed between the channel and the gate. As with an ordinary [[diode]], the arrow points from P to N, the direction of [[Electric current#Conventional current|conventional current]] when forward-biased. An English [[mnemonic]] is that the arrow of an N-channel device "points i'''n'''".

== Comparison with other transistors ==
At room temperature, JFET gate current (the reverse leakage of the gate-to-channel [[P-n junction|junction]]) is comparable to that of a [[MOSFET]] (which has insulating oxide between gate and channel), but much less than the base current of a [[bipolar junction transistor]].  The JFET has higher gain ([[transconductance]]) than the MOSFET, as well as lower [[flicker noise]], and is therefore used in some low-[[Noise (physics)|noise]], high input-impedance [[Operational amplifier|op-amps]]. Additionally the JFET is less susceptible to damage from static charge buildup.<ref>{{Cite web |last=Kopp |first=Emilie |date=2019-01-16 |title=What's the difference between a MOSFET and a JFET? |url=https://www.powerelectronictips.com/whats-difference-between-a-mosfet-jfet-faq/ |url-status=live |archive-url=https://web.archive.org/web/20210517223931/https://www.powerelectronictips.com/whats-difference-between-a-mosfet-jfet-faq/ |archive-date=2021-05-17 |access-date=2022-06-16 |website=Power Electronic Tips}}</ref>

== Mathematical model ==
=== Linear ohmic region ===
The current in N-JFET due to a small voltage ''V''<sub>DS</sub> (that is, in the ''linear'' or ''ohmic''<ref>{{cite web |title=What is the Ohmic Region of a FET Transistor |url=http://www.learningaboutelectronics.com/Articles/What-is-the-ohmic-region-of-a-FET-transistor |access-date=2020-12-13 |website=www.learningaboutelectronics.com |quote=ohmic region ... also called the linear region}}</ref> or ''triode region''<ref name=":2" />) is given by treating the channel as a rectangular bar of material of [[electrical conductivity]] <math>q N_d \mu_n</math>:<ref name="kumar">{{cite book |author=Balbir Kumar and Shail B. Jain |title=Electronic Devices and Circuits |date=2013 |publisher=PHI Learning Pvt. Ltd. |isbn=9788120348448 |pages=342–345 |url=https://books.google.com/books?id=jr5nAgAAQBAJ&pg=PA343}}</ref> 
: <math>I_\text{D} = \frac{bW}{L} q N_d \mu_n V_\text{DS},</math>
where
: ''I''<sub>D</sub> = drain–source current,
: ''b'' = channel thickness for a given gate voltage,
: ''W'' = channel width,
: ''L'' = channel length,
: ''q'' = electron charge = 1.6{{e|−19}} C,
: ''μ<sub>n</sub>'' = [[electron mobility]],
: ''N<sub>d</sub>'' = n-type doping (donor) concentration,
: ''V''<sub>P</sub> = pinch-off voltage.

Then the drain current in the ''linear region'' can be approximated as

: <math>I_\text{D} = \frac{bW}{L} q N_d \mu_n V_\text{DS} = \frac{aW}{L} q N_d \mu_n \left(1 - \sqrt{\frac{V_\text{GS}}{V_\text{P}}}\right) V_\text{DS}.</math>

In terms of <math>I_\text{DSS}</math>, the drain current can be expressed as{{cn|date=February 2014}}

: <math>I_\text{D} = \frac{2 I_\text{DSS}}{V_\text{P}^2} \left(V_\text{GS} - V_\text{P} - \frac{V_\text{DS}}{2}\right) V_\text{DS}.</math>

=== Constant-current region ===
The drain current in the ''saturation'' or ''active''<ref>{{cite web |title=Junction Field Effect Transistor |url=https://www.electronics-tutorials.ws/transistor/tran_5.html |website=Electronics Tutorials |quote=Saturation or Active Region}}</ref><ref name=":2" /> or ''pinch-off region''<ref>{{cite web |last=Scholberg |first=Kate |date=2017-03-23 |title=What is the meaning of "pinch-off region"? |url=https://webhome.phy.duke.edu/~schol/phy271/faqs/faq17/node7.html |quote=The "pinch-off region" (or "saturation region") refers to operation of a FET with <math>V_{ds}</math> more than a few volts.}}</ref> is often approximated in terms of gate bias as<ref name=kumar/>

: <math>I_\text{DS} = I_\text{DSS} \left(1 - \frac{V_\text{GS}}{V_\text{P}}\right)^2,</math>

where ''I''<sub>DSS</sub> is the saturation current at zero gate–source voltage, i.e. the maximum current that can flow through the FET from drain to source at any (permissible) drain-to-source voltage (see, e. g., the ''I''–''V'' characteristics diagram above).

In the ''saturation region'', the JFET drain current is most significantly affected by the gate–source voltage and barely affected by the drain–source voltage.

If the channel doping is uniform, such that the depletion region thickness will grow in proportion to the square root of the absolute value of the gate–source voltage, then the channel thickness ''b'' can be expressed in terms of the zero-bias channel thickness ''a'' as<ref>{{Cite web |last=Storr |first=Wayne |date=2013-09-03 |title=Junction Field Effect Transistor or JFET Tutorial |url=https://www.electronics-tutorials.ws/transistor/tran_5.html |access-date=2022-10-07 |website=Basic Electronics Tutorials |language=en}}</ref>{{Failed verification|date=October 2022}}

: <math>b = a \left(1 - \sqrt{\frac{V_\text{GS}}{V_\text{P}}}\right),</math>

where 
: ''V''<sub>P</sub> is the pinch-off voltage{{snd}} the gate–source voltage at which the channel thickness goes to zero,
: ''a'' is the channel thickness at zero gate–source voltage.

=== Transconductance ===
The transconductance for the junction FET is given by

: <math>g_\text{m} = \frac{2 I_\text{DSS}}{|V_\text{P}|} \left(1 - \frac{V_\text{GS}}{V_\text{P}}\right),</math>

where <math>V_\text{P}</math> is the pinchoff voltage, and ''I''<sub>DSS</sub> is the maximum drain current. This is also called <math>g_\text{fs}</math> or <math>y_\text{fs}</math> (for [[transadmittance]]).<ref>{{Cite news |last=Kirt Blattenberger RF Cafe |title=JFETS: How They Work, How to Use Them, May 1969 Radio-Electronics |language=en |url=https://www.rfcafe.com/references/radio-electronics/jfets-radio-electronics-may-1969.htm |access-date=2021-01-04 |quote='''y<sub>fs</sub>''' – Small-signal, common-source, forward transadmittance (sometimes called g<sub>fs</sub>-transconductance)}}</ref>

== See also ==
* [[Constant-current diode]]
* [[Fetron]]
* [[MOSFET]]
* [[MESFET]]

== References ==
{{Reflist}}

== External links ==
*{{Commons category inline}}
* [http://instrumentationlab.berkeley.edu/Lab4 Physics 111 Laboratory -- JFET Circuits I]
* [http://www-g.eng.cam.ac.uk/mmg/teaching/linearcircuits/jfet.html Interactive Explanation of n-channel JFET]

{{Transistor amplifiers}}
{{Electronic component}}
{{Portalbar|Electronics}}

[[Category:Transistor types]]
[[Category:Field-effect transistors]]