Junction Field-effect Transistor
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The junction-gate field-effect transistor (JFET) is one of the simplest types of
field-effect transistor The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs ( JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs con ...
. JFETs are three-terminal
semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
devices that can be used as electronically controlled
switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type ...
es or resistors, or to build amplifiers. Unlike bipolar junction transistors, JFETs are exclusively
voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge t ...
-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 processin ...
current.
Electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respecti ...
flows through a semiconducting channel between ''source'' and ''drain'' terminals. By applying a reverse bias
voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge t ...
to a ''gate'' terminal, the channel is '' pinched'', 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 movin ...
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 o ...
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 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 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 ...
,
Walter Houser Brattain Walter Houser Brattain (; February 10, 1902 – October 13, 1987) was an American physicist at Bell Labs who, along with fellow scientists John Bardeen and William Shockley, invented the point-contact transistor in December 1947. They shared t ...
, and
William Shockley William Bradford Shockley Jr. (February 13, 1910 – August 12, 1989) was an American physicist and inventor. He was the manager of a research group at Bell Labs that included John Bardeen and Walter Brattain. The three scientists were jointly ...
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 M. Ross. 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.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 Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal ...
(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 A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
material, doped to contain an abundance of positive charge carriers or holes (''p-type''), or of negative carriers or
electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary partic ...
s (''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 A garden hose, hosepipe, or simply hose is a flexible tube used to convey water. There are a number of common attachments available for the end of the hose, such as sprayers and sprinklers (which are used to concentrate water at one point or ...
. The flow of water through a hose can be controlled by squeezing it to reduce the cross section and the flow of
electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respecti ...
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 Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...
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 The threshold voltage, commonly abbreviated as Vth or VGS(th), of a field-effect transistor (FET) is the minimum gate-to-source voltage (VGS) that is needed to create a conducting path between the source and drain terminals. It is an important ...
'' 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, the arrow points from P to N, the direction of conventional current when forward-biased. An English
mnemonic A mnemonic ( ) device, or memory device, is any learning technique that aids information retention or retrieval (remembering) in the human memory for better understanding. Mnemonics make use of elaborative encoding, retrieval cues, and image ...
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) 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 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 bipola ...
. The JFET has higher gain ( transconductance) than the MOSFET, as well as lower flicker noise, and is therefore used in some low-
noise Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...
, 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 allows ...
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 mob ...
, : ''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 *
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


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