# JFET

Last updated
Type Electric current from source to drain in a p-channel JFET is restricted when a voltage is applied to the gate. Active drain, gate, source

The junction gate field-effect transistor (JFET or JUGFET) is one of the simplest types of field-effect transistor. [1] JFETs are three-terminal semiconductor devices that can be used as electronically-controlled switches, amplifiers, or voltage-controlled resistors.

## Contents

Unlike bipolar transistors, JFETs are exclusively voltage-controlled in that they do not need a biasing 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", so that the electric current is impeded or switched off completely. A JFET is usually ON when there is no voltage between its gate and source terminals. If a potential difference of the proper 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 carriers; and 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 less than that applied to the source, the current will be reduced (similarly in the p-type, if the voltage applied to the gate is greater than that applied to the source). A JFET has a large input impedance (sometimes on the order of 1010 ohms), which means that it has a negligible effect on external components or circuits connected to its gate.

## History

A succession of FET-like devices was patented by 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. [2] During 1940s, researchers John Bardeen, Walter Houser Brattain, and William Shockley were trying to build a FET, but failed in their repeated attempts to make a FET. 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 F. Dacey and Ian M. Ross. [3] 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 length. [3]

## Structure

The JFET is a long channel of semiconductor material, doped to contain an abundance of positive charge carriers or holes (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).

## Function

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. [4]

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 (VGS) of the gate-source junction. The pinch-off voltage (Vp) varies considerably, even among devices of the same type. For example, VGS(off) for the Temic J202 device varies from −0.8 V to −4 V. [5] Typical values vary from −0.3 V to −10 V.

To switch off an n-channel device requires a negative gate-source voltage (VGS). Conversely, to switch off a p-channel device requires positive VGS.

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.

Officially, the style of the symbol should show the component inside a circle[ according to whom? ] (representing the envelope of a discrete device). This is true in both the US and Europe. The symbol is usually drawn without the circle when drawing schematics of integrated circuits. More recently, the symbol is often drawn without its circle even for discrete devices.

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 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 (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.

## Mathematical model

The current in N-JFET due to a small voltage VDS (that is, in the linear ohmic region) is given by treating the channel as a rectangular bar of material of electrical conductivity ${\displaystyle qN_{d}\mu _{n}}$: [6]

${\displaystyle I_{\rm {D}}={\frac {bW}{L}}qN_{d}\mu _{n}V_{\rm {DS}}}$

where

ID = drain–source current
b = channel thickness for a given gate voltage
W = channel width
L = channel length
q = electron charge = 1.6 x 10−19 C
μn = electron mobility
Nd = n-type doping (donor) concentration.
VP = pinch-off voltage.

### Linear region

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

${\displaystyle I_{\rm {D}}={\frac {bW}{L}}qN_{d}{{\mu }_{n}}V_{DS}={\frac {aW}{L}}qN_{d}{{\mu }_{n}}\left(1-{\sqrt {\frac {V_{\rm {GS}}}{V_{\rm {P}}}}}\right)V_{\rm {DS}}}$

In terms of ${\displaystyle I_{\rm {DSS}}}$, the drain current can be expressed as:[ citation needed ]

${\displaystyle I_{\rm {D}}={\frac {2I_{\rm {DSS}}}{V_{\rm {P}}^{2}}}\left(V_{\rm {GS}}-V_{\rm {P}}-{\frac {V_{\rm {DS}}}{2}}\right)V_{\rm {DS}}}$

### Constant current region

The drain current in the saturation region is often approximated in terms of gate bias as: [6]

${\displaystyle I_{\rm {DS}}=I_{\rm {DSS}}\left(1-{\frac {V_{\rm {GS}}}{V_{\rm {P}}}}\right)^{2}}$

where

IDSS is the saturation current at zero gate–source voltage, i.e. the maximum current which 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:[ citation needed ]

${\displaystyle b=a\left(1-{\sqrt {\frac {V_{\rm {GS}}}{V_{\rm {P}}}}}\right)}$

where

VP 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 ${\displaystyle g_{m}={\frac {2I_{DSS}}{\left|{V_{P}}\right|}}\left({1-{\frac {V_{GS}}{V_{P}}}}\right)}$, where VP is the pinchoff voltage and IDSS is the maximum drain current.

## Related Research Articles

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals controls the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.

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), is a type of insulated-gate field-effect transistor (IGFET) that is fabricated by the controlled oxidation of a semiconductor, typically silicon. The voltage of the covered gate determines the electrical conductivity of the device; this ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. The MOSFET was invented by Egyptian engineer Mohamed M. Atalla and Korean engineer Dawon Kahng at Bell Labs in November 1959. It is the basic building block of modern electronics, and the most frequently manufactured device in history, with an estimated total of 13 sextillion (1.3 × 1022) MOSFETs manufactured between 1960 and 2018.

A current mirror is a circuit designed to copy a current through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. The current being "copied" can be, and sometimes is, a varying signal current. Conceptually, an ideal current mirror is simply an ideal inverting current amplifier that reverses the current direction as well. Or it can consist of a current-controlled current source (CCCS). The current mirror is used to provide bias currents and active loads to circuits. It can also be used to model a more realistic current source.

A p–n junction is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor. The "p" (positive) side contains an excess of holes, while the "n" (negative) side contains an excess of electrons in the outer shells of the electrically neutral atoms there. This allows electrical current to pass through the junction only in one direction. The p-n junction is created by doping, for example by ion implantation, diffusion of dopants, or by epitaxy. If two separate pieces of material were used, this would introduce a grain boundary between the semiconductors that would severely inhibit its utility by scattering the electrons and holes.

Transconductance, 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 reciprocal of resistance.

A MESFET is a field-effect transistor semiconductor device similar to a JFET with a Schottky (metal-semiconductor) junction instead of a p-n junction for a gate.

The threshold voltage, commonly abbreviated as Vth, of a field-effect transistor (FET) is the minimum gate-to-source voltage VGS (th) that is needed to create a conducting path between the source and drain terminals. It is an important scaling factor to maintain power efficiency.

In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region, space charge region or space charge layer, is an insulating region within a conductive, doped semiconductor material where the mobile charge carriers have been diffused away, or have been forced away by an electric field. The only elements left in the depletion region are ionized donor or acceptor impurities.

In mesoscopic physics, a Coulomb blockade (CB), named after Charles-Augustin de Coulomb's electrical force, is the decrease in electrical conductance at small bias voltages of a small electronic device comprising at least one low-capacitance tunnel junction. Because of the CB, the conductance of a device may not be constant at low bias voltages, but disappear for biases under a certain threshold, i.e. no current flows.

The cascode is a two-stage amplifier that consists of a common-emitter stage feeding into a common-base stage.

A power MOSFET is a specific type of metal–oxide–semiconductor field-effect transistor (MOSFET) designed to handle significant power levels. Compared to the other power semiconductor devices, such as an insulated-gate bipolar transistor (IGBT) or a thyristor, its main advantages are high switching speed and good efficiency at low voltages. It shares with the IGBT an isolated gate that makes it easy to drive. They can be subject to low gain, sometimes to a degree that the gate voltage needs to be higher than the voltage under control.

One of several short-channel effects in MOSFET scaling, channel length modulation (CLM) is a shortening of the length of the inverted channel region with increase in drain bias for large drain biases. The result of CLM is an increase in current with drain bias and a reduction of output resistance. Channel length modulation occurs in all field effect transistors, not just MOSFETs.

Drain-induced barrier lowering (DIBL) is a short-channel effect in MOSFETs referring originally to a reduction of threshold voltage of the transistor at higher drain voltages. In a classic planar field-effect transistor with a long channel, the bottleneck in channel formation occurs far enough from the drain contact that it is electrostatically shielded from the drain by the combination of the substrate and gate, and so classically the threshold voltage was independent of drain voltage. In short-channel devices this is no longer true: The drain is close enough to gate the channel, and so a high drain voltage can open the bottleneck and turn on the transistor prematurely.

A carbon nanotube field-effect transistor (CNTFET) refers to a field-effect transistor that utilizes a single carbon nanotube or an array of carbon nanotubes as the channel material instead of bulk silicon in the traditional MOSFET structure. First demonstrated in 1998, there have been major developments in CNTFETs since.

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.

Single-walled carbon nanotubes have the ability to conduct electricity. This conduction can be ballistic, diffusive, or based on scattering. When ballistic in nature conductance can be treated as if the electrons experience no scattering.

A FET amplifier is an amplifier that uses one or more field-effect transistors (FETs), particularly MOSFETs. The main advantage of a FET used for amplification is that it has very high input impedance and low output impedance.

The field-effect transistor (FET) is a type of transistor which uses an electric field to control the flow of current. FETs are devices with three terminals: source, gate, and drain. FETs control the flow of current by the application of a voltage to the gate, which in turn alters the conductivity between the drain and source.

A voltage-controlled resistor (VCR) is a three-terminal active device with one input port and two output ports. The input-port voltage controls the value of the resistor between the output ports. VCRs are most often built with field-effect transistors (FETs). Two types of FETs are often used: the JFET and the MOSFET. There are both floating-voltage controlled resistors and grounded floating resistors. Floating VCRs can be placed between two passive or active components. Grounded VCRs, the more common and less complicated design, require that one port of the voltage controlled resistor be grounded.

A single-electron transistor (SET) is a sensitive electronic device based on the Coulomb blockade effect. In this device the electrons flow through a tunnel junction between source/drain to a quantum dot. Moreover, the electrical potential of the island can be tuned by a third electrode, known as the gate, which is capacitively coupled to the island. The conductive island is sandwiched between two tunnel junctions, which are modeled by a capacitor and a resistor in parallel.

## References

1. Hall, John. "Discrete JFET" (PDF). linearsystems.com.
2. Grundmann, Marius (2010). The Physics of Semiconductors. Springer-Verlag. ISBN   978-3-642-13884-3.
3. Junction Field-Effect Devices, Semiconductor Devices for Power Conditioning, 1982
4. For a discussion of JFET structure and operation, see for example D. Chattopadhyay (2006). "§13.2 Junction field-effect transistor (JFET)". Electronics (fundamentals and applications). New Age International. pp. 269 ff. ISBN   978-8122417807.
5. Balbir Kumar and Shail B. Jain (2013). Electronic Devices and Circuits. PHI Learning Pvt. Ltd. pp. 342–345. ISBN   9788120348448.