# 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.

The field-effect transistor (FET) is an electronic device 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 semiconductor material has an electrical conductivity value falling between that of a conductor, such as metallic copper, and an insulator, such as glass. Its resistance falls as its temperature rises; metals are the opposite. Its conducting properties may be altered in useful ways by introducing impurities ("doping") into the crystal structure. Where two differently-doped regions exist in the same crystal, a semiconductor junction is created. The behavior of charge carriers which include electrons, ions and electron holes at these junctions is the basis of diodes, transistors and all modern electronics. Some examples of semiconductors are silicon, germanium, gallium arsenide, and elements near the so-called "metalloid staircase" on the periodic table. After silicon, gallium arsenide is the second most common semiconductor and is used in laser diodes, solar cells, microwave-frequency integrated circuits and others. Silicon is a critical element for fabricating most electronic circuits.

Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter.

## 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.

Biasing in electronics means establishing predetermined voltages or currents at various points of an electronic circuit for the purpose of establishing proper operating conditions in electronic components.

Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of electric charge: positive and negative. Like charges repel and unlike attract. An object with an absence of net charge is referred to as neutral. Early knowledge of how charged substances interact is now called classical electrodynamics, and is still accurate for problems that do not require consideration of quantum effects.

Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential between two points. The difference in electric potential between two points in a static electric field is defined as the work needed per unit of charge to move a test charge between the two points. In the International System of Units, the derived unit for voltage is named volt. In SI units, work per unit charge is expressed as joules per coulomb, where 1 volt = 1 joule per 1 coulomb. The official SI definition for volt uses power and current, where 1 volt = 1 watt per 1 ampere. This definition is equivalent to the more commonly used 'joules per coulomb'. Voltage or electric potential difference is denoted symbolically by V, but more often simply as V, for instance in the context of Ohm's or Kirchhoff's circuit laws.

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.

Julius Edgar Lilienfeld was an Austro-Hungarian American physicist and electical engineer, credited with the first patents on the field-effect transistor (FET) (1925) and electrolytic capacitor (1931). Because of his failure to publish articles in learned journals and because high-purity semiconductor materials were not available yet, his FET patent never achieved fame, causing confusion for later inventors.

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]

Heinrich Johann Welker was a German theoretical and applied physicist who invented the "transistron", a transistor made at Westinghouse independently of the first successful transistor made at Bell Laboratories. He did fundamental work in III-V compound semiconductors, and paved the way for microwave semiconductor elements and laser diodes.

John Bardeen was an American physicist and electrical 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 transistor; and again in 1972 with Leon N Cooper and John Robert Schrieffer for a fundamental theory of conventional superconductivity known as the BCS theory.

Walter Houser Brattain 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 the 1956 Nobel Prize in Physics for their invention. Brattain devoted much of his life to research on surface states.

## 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).

In semiconductor production, doping is the intentional introduction of impurities into an intrinsic semiconductor for the purpose of modulating its electrical, optical and structural properties. The doped material is referred to as an extrinsic semiconductor. A semiconductor doped to such high levels that it acts more like a conductor than a semiconductor is referred to as a degenerate semiconductor.

In physics, chemistry, and electronic engineering, an electron hole is the lack of an electron at a position where one could exist in an atom or atomic lattice. Since in a normal atom or crystal lattice the negative charge of the electrons is balanced by the positive charge of the atomic nuclei, the absence of an electron leaves a net positive charge at the hole's location. Holes are not actually particles, but rather quasiparticles; they are different from the positron, which is the antiparticle of the electron.

An ohmic contact is a non-rectifying electrical junction: a junction between two conductors that has a linear current–voltage (I-V) curve as with Ohm's law. Low resistance ohmic contacts are used to allow charge to flow easily in both directions between the two conductors, without blocking due to rectification or excess power dissipation due to voltage thresholds.

## 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.

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## 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.