Editing JFET (section)

== 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.