JFET P-Channel Labelled.svg
   HOME

TheInfoList



OR:

The junction-gate field-effect transistor (JFET) is one of the simplest types of field-effect transistor. JFETs are three-terminal semiconductor devices that can be used as electronically controlled switches or
resistors A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...
, or to build
amplifiers An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost the v ...
. Unlike
bipolar junction transistors A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipolar ...
, JFETs are exclusively voltage-controlled in that they do not need a
biasing In electronics, biasing is the setting of DC (direct current) operating conditions (current and voltage) of an active device in an amplifier. Many electronic devices, such as diodes, transistors and vacuum tubes, whose function is processing ...
current. Electric charge flows through a semiconducting channel between ''source'' and ''drain'' terminals. By applying a reverse bias voltage to a ''gate'' terminal, the channel is ''
pinched ''Pinched'' is a 1917 American short comedy film starring Harold Lloyd. A print of the film is held by the Museum of Modern Art, and it has been released on DVD. Like many American films of the time, ''Pinched'' was subject to cuts by city and ...
'', so that the
electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...
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
polarity Polarity may refer to: Science *Electrical polarity, direction of electrical current *Polarity (mutual inductance), the relationship between components such as transformer windings * Polarity (projective geometry), in mathematics, a duality of ord ...
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 In field-effect transistors (FETs), depletion mode and enhancement mode are two major transistor types, corresponding to whether the transistor is in an on state or an off state at zero gate–source voltage. Enhancement-mode MOSFETs (metal–o ...
devices, as they rely on the principle of a depletion region, which is devoid of majority charge carriers. The depletion region has to be closed to enable current to flow. JFETs can have an n-type or 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 In electronics, a common-drain amplifier, also known as a source follower, is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a voltage buffer. In this circuit (NMOS) the gate terminal of the t ...
configuration has a large input impedance (sometimes on the order of 1010  ohms), little current is drawn from circuits used as input to the gate.


History

A succession of FET-like devices was patented by
Julius Lilienfeld Julius Edgar Lilienfeld (April 18, 1882 – August 28, 1963) was an Austro-Hungarian, and later American (where he moved in 1921) physicist and electrical engineer, who was credited with the first patent on the field-effect (FET) (1925). Be ...
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. During the 1940s, researchers
John Bardeen John Bardeen (; May 23, 1908 – January 30, 1991) was an American physicist and engineer. He is the only person to be awarded the Nobel Prize in Physics twice: first in 1956 with William Shockley and Walter Brattain for the invention of the tran ...
, Walter Houser Brattain, and William Shockley were trying to build a FET, but failed in their repeated attempts. They discovered the
point-contact transistor The point-contact transistor was the first type of transistor to be successfully demonstrated. It was developed by research scientists John Bardeen and Walter Brattain at Bell Laboratories in December 1947. They worked in a group led by physicis ...
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 George may refer to: People * George (given name) * George (surname) * George (singer), American-Canadian singer George Nozuka, known by the mononym George * George Washington, First President of the United States * George W. Bush, 43rd President ...
and Ian M. Ross. Japanese engineers Jun-ichi Nishizawa and Y. Watanabe applied for a patent for a similar device in 1950 termed
static induction transistor The static induction transistor (SIT) is a type of field-effect transistor (FET) capable of high-speed and high-power operation, with low distortion and low noise. It is a vertical structure device with short multichannel. The device was original ...
(SIT). The SIT is a type of JFET with a short channel.Junction Field-Effect Devices
''Semiconductor Devices for Power Conditioning'', 1982.
High-speed, high-voltage switching with JFETs became technically feasible following the commercial introduction of Silicon carbide (SiC) wide-bandgap devices in 2008. Due to early difficulties in manufacturing — in particular, inconsistencies and low yield — 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.. 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.


Structure

The JFET is a long channel of semiconductor material, doped to contain an abundance of positive charge carriers or
holes A hole is an opening in or through a particular medium, usually a solid body. Holes occur through natural and artificial processes, and may be useful for various purposes, or may represent a problem needing to be addressed in many fields of en ...
(''p-type''), or of negative carriers or electrons (''n-type''). Ohmic contacts at each end form the source (S) and the drain (D). A 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

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 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 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: 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. For a discussion of JFET structure and operation, see for example 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''GS) of the gate–source junction. The ''pinch-off voltage'' (Vp) (also known as '' threshold voltage'' or ''cut-off voltage'') varies considerably, even among devices of the same type. For example, ''V''GS(off) for the Temic J202 device varies from to . Typical values vary from to . (Confusingly, the term ''pinch-off voltage'' is also used to refer to the ''V''DS value that separates the linear and saturation regions.) To switch off an n-channel device requires a negative gate–source voltage (''V''GS). Conversely, to switch off a p-channel device requires positive ''V''GS. 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

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. 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 A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A diode ...
, the arrow points from P to N, the direction of conventional current when forward-biased. An English mnemonic is that the arrow of an N-channel device "points in".


Comparison with other transistors

At room temperature, JFET gate current (the reverse leakage of the gate-to-channel
junction Junction may refer to: Arts and entertainment * ''Junction'' (film), a 2012 American film * Jjunction, a 2002 Indian film * Junction (album), a 1976 album by Andrew Cyrille * Junction (EP), by Basement Jaxx, 2002 * Junction (manga), or ''Hot ...
) is comparable to that of a
MOSFET The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
(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, high input-impedance op-amps. Additionally the JFET is less susceptible to damage from static charge buildup.


Mathematical model


Linear ohmic region

The current in N-JFET due to a small voltage ''V''DS (that is, in the ''linear'' or ''ohmic'' or ''triode region'') is given by treating the channel as a rectangular bar of material of
electrical conductivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allow ...
q N_d \mu_n: :I_\text = \frac q N_d \mu_n V_\text, where : ''I''D = drain–source current, : ''b'' = channel thickness for a given gate voltage, : ''W'' = channel width, : ''L'' = channel length, : ''q'' = electron charge = 1.6 C, : ''μn'' =
electron mobility In solid-state physics, the electron mobility characterises how quickly an electron can move through a metal or semiconductor when pulled by an electric field. There is an analogous quantity for holes, called hole mobility. The term carrier mobili ...
, : ''Nd'' = n-type doping (donor) concentration, : ''V''P = pinch-off voltage. Then the drain current in the ''linear region'' can be approximated as : I_\text = \frac q N_d \mu_n V_\text = \frac q N_d \mu_n \left(1 - \sqrt\right) V_\text. In terms of I_\text, the drain current can be expressed as : I_\text = \frac \left(V_\text - V_\text - \frac\right) V_\text.


Constant-current region

The drain current in the ''saturation'' or ''active'' or ''pinch-off region'' is often approximated in terms of gate bias as : I_\text = I_\text \left(1 - \frac\right)^2, where ''I''DSS 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 : b = a \left(1 - \sqrt\right), where : ''V''P is the pinch-off voltage 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 : g_\text = \frac \left(1 - \frac\right), where V_\text is the pinchoff voltage, and ''I''DSS is the maximum drain current. This is also called g_\text or y_\text (for
transadmittance Transconductance (for transfer conductance), also infrequently called mutual conductance, is the electrical characteristic relating the current through the output of a device to the voltage across the input of a device. Conductance is the reciproca ...
).


See also

*
Constant-current diode A constant-current diode is an electronic device that limits current to a maximal specified value for the device. It is known as a current-limiting diode (CLD) or current-regulating diode (CRD). It consists of an n-channel JFET with the gate sho ...
*
Fetron The Fetron was a range of solid-state, plug-compatible replacements for vacuum tubes (valves). Fetrons were manufactured by Teledyne Semiconductor from 1967; primarily as a low-maintenance and low-power swap-in to replace vacuum tubes, which wer ...
*
MOSFET The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
* MESFET


References


External links

*
Physics 111 Laboratory -- JFET Circuits I


{{Portalbar, Electronics Transistor types Field-effect transistors