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What are fet transistor used for?

5 Answer(s) Available
Answer # 1 #

FETs, Field Effect Transistors Includes: FET basics     FET specifications     JFET     MOSFET     Dual gate MOSFET     Power MOSFET     MESFET / GaAs FET     HEMT & PHEMT     FinFET technology     IGBT     Silicon carbide, SiC MOSFET     GaN FET / HEMT

The field effect transistor, FET is a key electronic component using within many areas of the electronics industry.

The FET used in many circuits constructed from discrete electronic components in areas from RF technology to power control and electronic switching to general amplification.

However the major use for the field effect transistor, FET is within integrated circuits. In this application FET circuits consume much lower levels of power than ICs using bipolar transistor technology. This enables the very large scale integrated circuits to operate. If bipolar technology was used the power consumption would be orders of magnitude greater and the power generated far too large to dissipate from the integrated circuit.

Apart from being used in integrated circuits, discrete versions of these semiconductor devices are available both as leaded electronic components and also as surface mount devices.

Before the first FETs were introduced into the electronic components market, the concept of these semiconductor devices had been known for a number of years. There had been many difficulties in realising this type of device and making it work.

Some of the early concepts for the field effect transistor were outlined in a paper by Lilienfield in 1926, and in another paper by Heil in 1935.

The next foundations were set in place during the 1940s at Bell Laboratories where the semiconductor research group was set up. This group investigated a number of areas pertaining to semiconductors and semiconductor technology, one of which was a device that would modulate the current flowing in a semiconductor channel buy placing an electric field close to it.

During these early experiments, the researchers were unable to make the idea work, turning their ideas to another idea and ultimately inventing another form of semiconductor electronics component: the bipolar transistor.

After this much of the semiconductor research was focussed on improving the bipolar transistor, and the idea for a field effect transistor was not fully investigated for some while. Now FETs are very widely used, providing the main active element in many integrated circuits. Without these electronic components electronics technology would be very different to what it is now.

The concept of the field effect transistor is based around the concept that charge on a nearby object can attract charges within a semiconductor channel. It essentially operates using an electric field effect - hence the name.

The FET consists of a semiconductor channel with electrodes at either end referred to as the drain and the source.

A control electrode called the gate is placed in very close proximity to the channel so that its electric charge is able to affect the channel.

In this way, the gate of the FET controls the flow of carriers (electrons or holes) flowing from the source to drain. It does this by controlling the size and shape of the conductive channel.

The semiconductor channel where the current flow occurs may be either P-type or N-type. This gives rise to two types or categories of FET known as P-Channel and N-Channel FETs.

In addition to this, there are two further categories. Increasing the voltage on the gate can either deplete or enhance the number of charge carriers available in the channel. As a result there are enhancement mode FET and depletion mode FETs.

As it is only the electric field that controls the current flowing in the channel, the device is said to be voltage operated and it has a high input impedance, usually many megohms. This can be a distinct advantage over the bipolar transistor that is current operated and has a much lower input impedance.

Field effect transistors are widely used in all forms of electronic circuit designs from those used in circuits with discrete electronic components, to those employed in integrated circuits.

As the field effect transistor is a voltage operated semiconductor device rather than a current device like the bipolar transistor, this means that some aspects of the circuit are very different: the bias arrangements in particular. However electronic circuit design with FETs is relatively easy - it is just a bit different to that using bipolar transistors.

Using FETs, circuits like voltage amplifiers, buffers or current followers, oscillators, filters and many more can all be designed, and the circuit designs are very similar to those for bipolar transistors and even thermionic valves / vacuum tubes, although the bias arrangements are different. Interestingly valves / tubes are also voltage operated devices, and therefore their circuit designs are very similar, even in terms of the bias arrangements.

There are many ways to define the different types of FET that are available. The different types mean that during the electronic circuit design, there is a choice of the right electronic component for the circuit. By selecting the right device it is possible to obtain the best performance for the given circuit.

FETs may be categorised in a number of ways, but some of the major types of FET can be covered in the tree diagram below.

There are many different types of FET on the market for which there are various names. Some of the major categories are delayed below.

Although there are some other types of field effect transistor that may be seen in the literature, often these types are trade names for a particular technology and they are variants of some of the FET types listed above.

Apart from selecting a particular type of field effect transistor for any given circuit design, it is also necessary to understand the different specifications. In this way it is possible to ensure that the FET will operate to the required performance parameters.

FET specifications include everything from the maximum voltages and currents permissible to the capacitance levels and the transconductance. These all play a part in determining whether any particular FET is suitable for a given circuit or application.

Field affect transistor technology can be used in a number of areas where bipolar transistors are not as suitable: each of these semiconductor devices has its own advantages and disadvantages, and can be used to great effect in many circuits. The field effect transistor has a very high input impedance and is a voltage driven device and this opens it up to being used in many areas.

Bertha Leder
Answer # 2 #

FETs are three-terminal devices with a source, gate, and drain. The application of a voltage to the gate, which modifies the conductivity between the drain and source, controls the flow of current in FETs.

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The first patent for FET transistors was filed by Julias Edgar in 1926. Since then much development has taken place. Another patent was filed by Oskar Heil in 1934. The junction gate that is used in field-effect transistors was created at the Bell Labs by William Shockley. Many other advancements in FET Transistors have been made over the years.

The Fet transistor is a voltage-operated device in which the voltage applied is used to control the current flowing. It is also known by the name unipolar transistor as they undergo an operation of a single-carrier type. The input impedance is high in all forms and types of FET. The conductivity is always regulated with the help of applied voltage from the field-effect transistor’s terminal. Moreover, the density of the carrier charge affects conductivity.

A FET transistor is a device with three major components: Source, Drain, and Gate. The source is one of the terminals of the FET transistor through which most of the carriers enter the bar. The Drain is the second terminal through which the majority of carriers lead the bar. The Gate has two terminals that are internally connected with each other.

Since the gate in a FET transistor is reverse biased, the gate current is practically zero. The drain supply is connected to the source terminal leading to the electrons flow which provides the necessary carriers.

There is another subdivision of FET Transistors. In one of the types, the current is taken up primarily by the majority carriers and is therefore called majority charge carrier devices. There are minority charge carrier devices, as well,  in which the current flow is primarily due to minority carriers.

The two terminals, source, and gate have a potential between them which in turn has the conductivity of the channel as a function of it. The three terminals i.e. source, drain, and gate are there for every FET Transistor. The function of the gate terminal is similar to the gate in real life as the gate can open and close and can either choose to permit the passage of electrons or stop them altogether.

1.Junction Field Effect Transistor (JFET)

The Junction FET transistor is a form of field-effect transistor that can be used to control a switch electrically. Between the sources and the drain terminals, electric energy travels through an active channel.

The channel is strained and the electric current is switched off by supplying a reverse bias voltage to the gate terminal.

Working Principle:

The working of these JFETs is based on the channels that form between the terminals. Either an n-type or a p-type channel can be used. It's called an n-channel JFET because it has an n-type channel, and it's called a p-channel JFET because it has a p-type channel.

FET transistors are made in the same way as N-P-N and P-N-P transistors are made in BJT (Bipolar Junction Transistor). These JFETs have a channel that can be either n or p-type.

That is how an n-channel JFET operates. Only a change in the polarities of the supplies causes the FET to operate as a p-channel JFET.

2.Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

MOSFETs work by applying a voltage to channels that already exist or form. MOSFETs are classified into two types based on their operation modes:

In the enhancement mode, the gate voltage induces the channel, whereas, in the depletion mode, the MOSFET operates owing to the existing channel.

There are two types of MOSFET depletion models: n-type and p-type. The only difference is the substrate deposition. The formation of the depletion zone is caused by a concentration of carriers that are preferred by the majority. Conductivity is affected by the width of the depletion.

A channel is formed in the enhancement mode when a voltage applied to the gate terminal exceeds a threshold voltage. It could be n-type for a P-type substrate and p-type for an N-type substrate. The enhancement mode is classified as N-type Enhancement MOSFET or P-type Enhancement MOSFET based on the channel formation. MOSFETs of the enhancement type are more commonly used than those of the depletion type.

The main difference between the two major types of FET transistors - JFET and MOSFET- is that JFET (Junction Field Effect Transistor) is a three-terminal semiconductor device while MOSFET (Metal oxide semiconductor field-effect transistor) is a four-terminal semiconductor device. JFET can only operate in the depletion mode. While MOSFET can operate in the enhancement as well as the depletion mode. The input impedance is higher in MOSFET making them more resistive. In comparison to the price, MOSFET is more expensive than JFET.

Due to high input impedance, FET transistors are commonly used in and as input amplifiers in electronic voltmeters, oscilloscopes, and other measuring devices. They also occupy little space which makes them more efficient for other devices.

The article covers some important and key characteristics of FET Transistors. This foundational knowledge can be further used in understanding more concepts related to electricity and current. The definition of FET, types of FET, and how it regulates the circuits are the key highlights of this article.

Karthik Damle
Answer # 3 #

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An FET is a voltage-controlled device. This means that its output current is controlled by the voltage we apply to its gate terminal.

FETs have very high input impedance, which means they do not load down the signal source and can be used as buffer amplifiers. Using FETs as buffer amplifiers can help prevent signal distortion and improve the overall quality of the circuit’s output. Additionally, FETs are power-efficient, which makes them an attractive choice for battery-powered devices.

FETs are unipolar devices, which means they use only one type of charge carrier (electrons or holes) to control the current flow. The alternative to a unipolar device is a bipolar device. Unlike a unipolar device like an FET, a bipolar device such as a Bipolar Junction Transistor (BJT) uses both electrons and holes to control the current flow. Bipolar devices have a high current gain and can handle higher power levels, which makes them suitable for power amplification applications.

The source, the drain and the gate are an FET’s three terminals. The source and drain are connected to the channel, while the gate controls the flow of current through the channel.

Related ReadingNMOS Transistors and PMOS Transistors Explained

We can control the conductivity of the channel in an FET by the voltage we apply to the gate. In an n-channel FET, a positive voltage applied to the gate will attract electrons to the channel and increase its conductivity. In a p-channel FET, a negative voltage applied to the gate will attract holes to the channel and increase its conductivity.

In a JFET, the channel consists of a semiconductor material and the channel has two regions at each end. These are known as the source and the drain terminals. The gate is a PN junction that’s formed perpendicular to the channel. The gate terminal is biased in reverse. This creates a depletion region that controls the width of the channel. When we apply a voltage to the gate, the depletion region widens, thereby reducing the channel width and the current flowing through it.

Similar to JFETs, in MOSFETs the channel is also formed by a semiconductor material and it has two regions at either end, known as the source and drain terminals. In a MOSFET however, the gate is separated from the channel by a thin insulating layer that typically consists of silicon dioxide. As soon as a voltage is applied to the gate, it creates an electric field that attracts or repels charge carriers in the channel, depending on the voltage’s polarity. This process controls the width of the channel and the flow of current between the source and drain terminals.

MOSFETs can be further classified into two subtypes: enhancement-mode and depletion-mode MOSFETs.

In enhancement-mode MOSFETs, the channel is normally off and you must apply a positive voltage to the gate in order to turn it on.

In depletion-mode MOSFETs, the channel is normally on and you must apply a negative voltage to the gate to turn it off.

FETs have several advantages over other types of transistors, which make them popular in a variety of electronic applications.

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Timothée Lopez
Theatre Practitioner
Answer # 4 #

FET uses the voltage applied to its input terminal (called the Gate), to control the current flowing from the source to drain, making the Field Effect Transistor a “Voltage” operated device. FETs are extensively used in Integrated Circuits (ICs) due to their compact size and significantly lower power consumption.

Mindy DeKnight
Holistic Nursing
Answer # 5 #

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 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.

FETs are also known as unipolar transistors since they involve single-carrier-type operation. That is, FETs use either electrons (n-channel) or holes (p-channel) as charge carriers in their operation, but not both. Many different types of field effect transistors exist. Field effect transistors generally display very high input impedance at low frequencies. The most widely used field-effect transistor is the MOSFET (metal–oxide–semiconductor field-effect transistor).

The concept of a field-effect transistor (FET) was first patented by the Austro-Hungarian born physicist Julius Edgar Lilienfeld in 1925 and by Oskar Heil in 1934, but they were unable to build a working practical semiconducting device based on the concept. The transistor effect was later observed and explained by John Bardeen and Walter Houser Brattain while working under William Shockley at Bell Labs in 1947, shortly after the 17-year patent expired. Shockley initially attempted to build a working FET by trying to modulate the conductivity of a semiconductor, but was unsuccessful, mainly due to problems with the surface states, the dangling bond, and the germanium and copper compound materials. In the course of trying to understand the mysterious reasons behind their failure to build a working FET, it led to Bardeen and Brattain instead inventing the point-contact transistor in 1947, which was followed by Shockley's bipolar junction transistor in 1948.

The first FET device to be successfully built was the junction field-effect transistor (JFET). A JFET was first patented by Heinrich Welker in 1945. The static induction transistor (SIT), a type of JFET with a short channel, was invented by Japanese engineers Jun-ichi Nishizawa and Y. Watanabe in 1950. Following Shockley's theoretical treatment on the JFET in 1952, a working practical JFET was built by George C. Dacey and Ian M. Ross in 1953. However, the JFET still had issues affecting junction transistors in general. Junction transistors were relatively bulky devices that were difficult to manufacture on a mass-production basis, which limited them to a number of specialised applications. The insulated-gate field-effect transistor (IGFET) was theorized as a potential alternative to junction transistors, but researchers were unable to build working IGFETs, largely due to the troublesome surface state barrier that prevented the external electric field from penetrating into the material. By the mid-1950s, researchers had largely given up on the FET concept, and instead focused on bipolar junction transistor (BJT) technology.

The foundations of MOSFET technology were laid down by the work of William Shockley, John Bardeen and Walter Brattain. Shockley independently envisioned the FET concept in 1945, but he was unable to build a working device. The next year Bardeen explained his failure in terms of surface states. Bardeen applied the theory of surface states on semiconductors (previous work on surface states was done by Shockley in 1939 and Igor Tamm in 1932) and realized that the external field was blocked at the surface because of extra electrons which are drawn to the semiconductor surface. Electrons become trapped in those localized states forming an inversion layer. Bardeen's hypothesis marked the birth of surface physics. Bardeen then decided to make use of an inversion layer instead of the very thin layer of semiconductor which Shockley had envisioned in his FET designs. Based on his theory, in 1948 Bardeen patented the progenitor of MOSFET, an insulated-gate FET (IGFET) with an inversion layer. The inversion layer confines the flow of minority carriers, increasing modulation and conductivity, although its electron transport depends on the gate's insulator or quality of oxide if used as an insulator, deposited above the inversion layer. Bardeen's patent as well as the concept of an inversion layer forms the basis of CMOS technology today. In 1976 Shockley described Bardeen's surface state hypothesis "as one of the most significant research ideas in the semiconductor program".

After Bardeen's surface state theory the trio tried to overcome the effect of surface states. In late 1947, Robert Gibney and Brattain suggested the use of electrolyte placed between metal and semiconductor to overcome the effects of surface states. Their FET device worked, but amplification was poor. Bardeen went further and suggested to rather focus on the conductivity of the inversion layer. Further experiments led them to replace electrolyte with a solid oxide layer in the hope of getting better results. Their goal was to penetrate the oxide layer and get to the inversion layer. However, Bardeen suggested they switch from silicon to germanium and in the process their oxide got inadvertently washed off. They stumbled upon a completely different transistor, the point-contact transistor. Lillian Hoddeson argues that "had Brattain and Bardeen been working with silicon instead of germanium they would have stumbled across a successful field effect transistor".

By the end of the first half of the 1950s, following theoretical and experimental work of Bardeen, Brattain, Kingston, Morrison and others, it became more clear that there were two types of surface states. Fast surface states were found to be associated with the bulk and a semiconductor/oxide interface. Slow surface states were found to be associated with the oxide layer because of adsorption of atoms, molecules and ions by the oxide from the ambient. The latter were found to be much more numerous and to have much longer relaxation times. At the time Philo Farnsworth and others came up with various methods of producing atomically clean semiconductor surfaces.

In 1955, Carl Frosch and Lincoln Derrick accidentally covered the surface of silicon wafer with a layer of silicon dioxide. They showed that oxide layer prevented certain dopants into the silicon wafer, while allowing for others, thus discovering the passivating effect of oxidation on the semiconductor surface. Their further work demonstrated how to etch small openings in the oxide layer to diffuse dopants into selected areas of the silicon wafer. In 1957, they published a research paper and patented their technique summarizing their work. The technique they developed is known as oxide diffusion masking, which would later be used in the fabrication of MOSFET devices. At Bell Labs, the importance of Frosch's technique was immediately realized. Results of their work circulated around Bell Labs in the form of BTL memos before being published in 1957. At Shockley Semiconductor, Shockley had circulated the preprint of their article in December 1956 to all his senior staff, including Jean Hoerni.

In 1955, Ian Munro Ross filed a patent for a FeFET or MFSFET. Its structure was like that of a modern inversion channel MOSFET, but ferroelectric material was used as a dielectric/insulator instead of oxide. He envisioned it as a form of memory, years before the floating gate MOSFET. In February 1957, John Wallmark filed a patent for FET in which germanium monoxide was used as a gate dielectric, but he didn't pursue the idea. In his other patent filed the same year he described a double gate FET. In March 1957, in his laboratory notebook, Ernesto Labate, a research scientist at Bell Labs, conceived of a device similar to the later proposed MOSFET, although Labate's device didn't explicitly use silicon dioxide as an insulator.

A breakthrough in FET research came with the work of Egyptian engineer Mohamed Atalla in the late 1950s. In 1958 he presented experimental work which showed that growing thin silicon oxide on clean silicon surface leads to neutralization of surface states. This is known as surface passivation, a method that became critical to the semiconductor industry as it made mass-production of silicon integrated circuits possible.

The metal–oxide–semiconductor field-effect transistor (MOSFET) was then invented by Mohamed Atalla and Dawon Kahng in 1959. The MOSFET largely superseded both the bipolar transistor and the JFET, and had a profound effect on digital electronic development. With its high scalability, and much lower power consumption and higher density than bipolar junction transistors, the MOSFET made it possible to build high-density integrated circuits. The MOSFET is also capable of handling higher power than the JFET. The MOSFET was the first truly compact transistor that could be miniaturised and mass-produced for a wide range of uses. The MOSFET thus became the most common type of transistor in computers, electronics, and communications technology (such as smartphones). The US Patent and Trademark Office calls it a "groundbreaking invention that transformed life and culture around the world".

CMOS (complementary MOS), a semiconductor device fabrication process for MOSFETs, was developed by Chih-Tang Sah and Frank Wanlass at Fairchild Semiconductor in 1963. The first report of a floating-gate MOSFET was made by Dawon Kahng and Simon Sze in 1967. A double-gate MOSFET was first demonstrated in 1984 by Electrotechnical Laboratory researchers Toshihiro Sekigawa and Yutaka Hayashi. FinFET (fin field-effect transistor), a type of 3D non-planar multi-gate MOSFET, originated from the research of Digh Hisamoto and his team at Hitachi Central Research Laboratory in 1989.

FETs can be majority-charge-carrier devices, in which the current is carried predominantly by majority carriers, or minority-charge-carrier devices, in which the current is mainly due to a flow of minority carriers. The device consists of an active channel through which charge carriers, electrons or holes, flow from the source to the drain. Source and drain terminal conductors are connected to the semiconductor through ohmic contacts. The conductivity of the channel is a function of the potential applied across the gate and source terminals.

The FET's three terminals are:

All FETs have source, drain, and gate terminals that correspond roughly to the emitter, collector, and base of BJTs. Most FETs have a fourth terminal called the body, base, bulk, or substrate. This fourth terminal serves to bias the transistor into operation; it is rare to make non-trivial use of the body terminal in circuit designs, but its presence is important when setting up the physical layout of an integrated circuit. The size of the gate, length L in the diagram, is the distance between source and drain. The width is the extension of the transistor, in the direction perpendicular to the cross section in the diagram (i.e., into/out of the screen). Typically the width is much larger than the length of the gate. A gate length of 1 µm limits the upper frequency to about 5 GHz, 0.2 µm to about 30 GHz.

The names of the terminals refer to their functions. The gate terminal may be thought of as controlling the opening and closing of a physical gate. This gate permits electrons to flow through or blocks their passage by creating or eliminating a channel between the source and drain. Electron-flow from the source terminal towards the drain terminal is influenced by an applied voltage. The body simply refers to the bulk of the semiconductor in which the gate, source and drain lie. Usually the body terminal is connected to the highest or lowest voltage within the circuit, depending on the type of the FET. The body terminal and the source terminal are sometimes connected together since the source is often connected to the highest or lowest voltage within the circuit, although there are several uses of FETs which do not have such a configuration, such as transmission gates and cascode circuits.

Unlike BJTs, the vast majority of FETs are electrically symmetrical. The source and drain terminals can thus be interchanged in practical circuits with no change in operating characteristics or function. This can be confusing when FET's appear to be connected "backwards" in schematic diagrams and circuits because the physical orientation of the FET was decided for other reasons, such as printed circuit layout considerations.

The FET controls the flow of electrons (or electron holes) from the source to drain by affecting the size and shape of a "conductive channel" created and influenced by voltage (or lack of voltage) applied across the gate and source terminals. (For simplicity, this discussion assumes that the body and source are connected.) This conductive channel is the "stream" through which electrons flow from source to drain.

In an n-channel "depletion-mode" device, a negative gate-to-source voltage causes a depletion region to expand in width and encroach on the channel from the sides, narrowing the channel. If the active region expands to completely close the channel, the resistance of the channel from source to drain becomes large, and the FET is effectively turned off like a switch (see right figure, when there is very small current). This is called "pinch-off", and the voltage at which it occurs is called the "pinch-off voltage". Conversely, a positive gate-to-source voltage increases the channel size and allows electrons to flow easily (see right figure, when there is a conduction channel and current is large).

In an n-channel "enhancement-mode" device, a conductive channel does not exist naturally within the transistor, and a positive gate-to-source voltage is necessary to create one. The positive voltage attracts free-floating electrons within the body towards the gate, forming a conductive channel. But first, enough electrons must be attracted near the gate to counter the dopant ions added to the body of the FET; this forms a region with no mobile carriers called a depletion region, and the voltage at which this occurs is referred to as the threshold voltage of the FET. Further gate-to-source voltage increase will attract even more electrons towards the gate which are able to active channel from source to drain; this process is called inversion.

In a p-channel "depletion-mode" device, a positive voltage from gate to body widens the depletion layer by forcing electrons to the gate-insulator/semiconductor interface, leaving exposed a carrier-free region of immobile, positively charged acceptor ions.

Conversely, in a p-channel "enhancement-mode" device, a conductive region does not exist and negative voltage must be used to generate a conduction channel.

For either enhancement- or depletion-mode devices, at drain-to-source voltages much less than gate-to-source voltages, changing the gate voltage will alter the channel resistance, and drain current will be proportional to drain voltage (referenced to source voltage). In this mode the FET operates like a variable resistor and the FET is said to be operating in a linear mode or ohmic mode.

If drain-to-source voltage is increased, this creates a significant asymmetrical change in the shape of the channel due to a gradient of voltage potential from source to drain. The shape of the inversion region becomes "pinched-off" near the drain end of the channel. If drain-to-source voltage is increased further, the pinch-off point of the channel begins to move away from the drain towards the source. The FET is said to be in saturation mode; although some authors refer to it as active mode, for a better analogy with bipolar transistor operating regions. The saturation mode, or the region between ohmic and saturation, is used when amplification is needed. The in-between region is sometimes considered to be part of the ohmic or linear region, even where drain current is not approximately linear with drain voltage.

Even though the conductive channel formed by gate-to-source voltage no longer connects source to drain during saturation mode, carriers are not blocked from flowing. Considering again an n-channel enhancement-mode device, a depletion region exists in the p-type body, surrounding the conductive channel and drain and source regions. The electrons which comprise the channel are free to move out of the channel through the depletion region if attracted to the drain by drain-to-source voltage. The depletion region is free of carriers and has a resistance similar to silicon. Any increase of the drain-to-source voltage will increase the distance from drain to the pinch-off point, increasing the resistance of the depletion region in proportion to the drain-to-source voltage applied. This proportional change causes the drain-to-source current to remain relatively fixed, independent of changes to the drain-to-source voltage, quite unlike its ohmic behavior in the linear mode of operation. Thus, in saturation mode, the FET behaves as a constant-current source rather than as a resistor, and can effectively be used as a voltage amplifier. In this case, the gate-to-source voltage determines the level of constant current through the channel.

FETs can be constructed from various semiconductors, out of which silicon is by far the most common. Most FETs are made by using conventional bulk semiconductor processing techniques, using a single crystal semiconductor wafer as the active region, or channel.

Among the more unusual body materials are amorphous silicon, polycrystalline silicon or other amorphous semiconductors in thin-film transistors or organic field-effect transistors (OFETs) that are based on organic semiconductors; often, OFET gate insulators and electrodes are made of organic materials, as well. Such FETs are manufactured using a variety of materials such as silicon carbide (SiC), gallium arsenide (GaAs), gallium nitride (GaN), and indium gallium arsenide (InGaAs).

In June 2011, IBM announced that it had successfully used graphene-based FETs in an integrated circuit. These transistors are capable of about 2.23 GHz cutoff frequency, much higher than standard silicon FETs.

The channel of a FET is doped to produce either an n-type semiconductor or a p-type semiconductor. The drain and source may be doped of opposite type to the channel, in the case of enhancement mode FETs, or doped of similar type to the channel as in depletion mode FETs. Field-effect transistors are also distinguished by the method of insulation between channel and gate. Types of FETs include:

Field-effect transistors have high gate-to-drain current resistance, of the order of 100 MΩ or more, providing a high degree of isolation between control and flow. Because base current noise will increase with shaping time, a FET typically produces less noise than a bipolar junction transistor (BJT), and is found in noise-sensitive electronics such as tuners and low-noise amplifiers for VHF and satellite receivers. It is relatively immune to radiation. It exhibits no offset voltage at zero drain current and makes an excellent signal chopper. It typically has better thermal stability than a BJT.

Because the FETs are controlled by gate charge, once the gate is closed or open, there is no additional power draw, as there would be with a bipolar junction transistor or with non-latching relays in some states. This allows extremely low-power switching, which in turn allows greater miniaturization of circuits because heat dissipation needs are reduced compared to other types of switches.

A field-effect transistor has a relatively low gain–bandwidth product compared to a bipolar junction transistor. MOSFETs are very susceptible to overload voltages, thus requiring special handling during installation. The fragile insulating layer of the MOSFET between the gate and the channel makes it vulnerable to electrostatic discharge or changes to threshold voltage during handling. This is not usually a problem after the device has been installed in a properly designed circuit.

FETs often have a very low "on" resistance and have a high "off" resistance. However, the intermediate resistances are significant, and so FETs can dissipate large amounts of power while switching. Thus, efficiency can put a premium on switching quickly, but this can cause transients that can excite stray inductances and generate significant voltages that can couple to the gate and cause unintentional switching. FET circuits can therefore require very careful layout and can involve trades between switching speed and power dissipation. There is also a trade-off between voltage rating and "on" resistance, so high-voltage FETs have a relatively high "on" resistance and hence conduction losses.

Field-effect transistors are relatively robust, especially when operated within the temperature and electrical limitations defined by the manufacturer (proper derating). However, modern FET devices can often incorporate a body diode. If the characteristics of the body diode are not taken into consideration, the FET can experience slow body diode behavior, where a parasitic transistor will turn on and allow high current to be drawn from drain to source when the FET is off.

The most commonly used FET is the MOSFET. The CMOS (complementary metal oxide semiconductor) process technology is the basis for modern digital integrated circuits. This process technology uses an arrangement where the (usually "enhancement-mode") p-channel MOSFET and n-channel MOSFET are connected in series such that when one is on, the other is off.

In FETs, electrons can flow in either direction through the channel when operated in the linear mode. The naming convention of drain terminal and source terminal is somewhat arbitrary, as the devices are typically (but not always) built symmetrical from source to drain. This makes FETs suitable for switching analog signals between paths (multiplexing). With this concept, one can construct a solid-state mixing board, for example. FET is commonly used as an amplifier. For example, due to its large input resistance and low output resistance, it is effective as a buffer in common-drain (source follower) configuration.

IGBTs are used in switching internal combustion engine ignition coils, where fast switching and voltage blocking capabilities are important.

Source-gated transistors are more robust to manufacturing and environmental issues in large-area electronics such as display screens, but are slower in operation than FETs.

Cote Pray
Set Designer