Field Effect Transistor (FET): Characteristics, Types & Uses


Field Effect Transistor (FET)

The field-effect transistor is a type of transistor that regulates the flow of current in a semiconductor by using an electric field. Source, gate, and drain are the three terminals on FETs.

Devices containing FETs (JFETs or MOSFETs) have three terminals: the source, gate, and drain. By applying a voltage to the gate, which in turn changes the conductivity between the drain and source, FETs may regulate the flow of current.

Since FETs operate using a single carrier type, they are often referred to as unipolar transistors. In other words, FETs operate using either electrons (n-channel) or holes (p-channel), but not both. Field effect transistors come in a wide range of varieties. At low frequencies, field effect transistors typically exhibit extremely high input impedance. The MOSFET is the most popular type of field-effect transistor (metal-oxide-semiconductor field-effect transistor).

FET Characteristics

FETs can be either majority-charge-carrier devices, where the majority carriers carry the majority of the current, or minority-charge-carrier devices, where the minority carriers carry the majority of the current. Charge carriers, like as electrons or holes, go from the source to the drain through the device's active channel. The semiconductor is coupled to the source and drain terminal wires via ohmic connections. The potential applied between the gate and source terminals determines the channel's conductivity.

Field Effect Transistor Terminals


The three terminals of the FET are:

the channel's source (S), via which the carriers enter. Drain (D), via which the carriers exit the channel, is conventionally marked as IS, which stands for current entering the channel at S. Traditionally, ID identifies the current entering the channel at D. Gate (G), the terminal that modifies the channel conductivity, is the drain-to-source voltage, or VDS. G can be made to respond to voltage, which controls ID.

The emitter, collector, and base of BJTs are nearly equivalent to the source, drain, and gate terminals of all FETs. The body, base, bulk, or substrate is the fourth terminal present in the majority of FETs. It is uncommon to make a non-trivial use of the body terminal in circuit designs, but its presence is crucial when setting up the physical layout of an integrated circuit. This fourth terminal is used to bias the transistor into operation. The distance between the source and drain is represented by the length L of the gate in the diagram. The width is the transistor's expansion into or out of the screen, or in other words, in a direction perpendicular to the diagram's cross section. Usually, the breadth of the gate is significantly more than its length. The upper frequency is restricted to approximately 5 GHz with a 0.2 m gate length, and to approximately 30 GHz.

The names of the terminals are descriptive of what they do. One could imagine the gate terminal as having control over a physical gate's opening and closing. This gate alters the path between the source and drain so that it either allows electrons to pass through or prevents them from doing so. An applied voltage affects how electrons move from the source terminal to the drain terminal. The term "body" simply denotes the region of the semiconductor that contains the gate, source, and drain. Depending on the kind of FET, the body terminal is often linked to either the highest or lowest voltage inside the circuit. Although there are many applications for FETs that do not have this configuration, such as transmission gates and cascode circuits, it is occasionally necessary to connect the body terminal and the source terminal together because the source is frequently connected to the highest or lowest voltage within the circuit.

The vast majority of FETs are electrically symmetrical, unlike BJTs. Thus, in real circuits, the source and drain terminals can be switched without affecting the operation or functionality. Because of other factors, such as printed circuit layout constraints, the physical orientation of the FET may appear to be linked "backwards" in schematic designs and circuits, which can be confusing.

FET Material Compositions

A variety of semiconductors, including silicon, which is by far the most popular, can be used to make FETs. The majority of FETs are produced using traditional bulk semiconductor fabrication methods, with the active region or channel being a single crystal semiconductor wafer.

Amorphous silicon, polycrystalline silicon, or other amorphous semiconductors used in thin-film transistors or organic field-effect transistors (OFETs) based on organic semiconductors are among the more unusual body materials. Gate insulators and electrodes for OFETs are frequently made of organic materials as well. A variety of materials, including silicon carbide (SiC), gallium arsenide (GaAs), gallium nitride (GaN), and indium gallium arsenide, are used to make these FETs.

IBM declared that it had successfully implemented graphene-based FETs in an integrated circuit in June 2011. These transistors have a cutoff frequency of about 2.23 GHz, which is significantly higher than that of conventional silicon FETs.

FET Types

Either an n-type semiconductor or a p-type semiconductor is created by doping the channel of a FET. In enhancement mode FETs, the drain and source may be doped in the opposite direction of the channel, or they may be doped in the same direction as the channel in depletion mode FETs. The way that the channel and gate are isolated from one another is another feature that sets field-effect transistors apart. Various FET types include:

An insulator (usually SiO2) is used by the MOSFET (metal-oxide-semiconductor field-effect transistor) between the gate and the body. This FET type is by far the most prevalent.

A MOSFET with two insulated gates is known as a DGMOSFET (dual-gate MOSFET) or DGMOS.

An instrument for controlling power is an IGBT (insulated-gate bipolar transistor). Its principal conduction channel resembles that of a bipolar MOSFET, and its structure is similar to a MOSFET. These are frequently utilised for functioning in the 200–3000 V drain–source voltage range. The preferred device for drain-to-source voltages between 1 and 200 V is still a power MOSFET.

Field-effect transistors (FETs) of the JLNT (Junctionless nanowire transistor) type have a channel made of one or more nanowires and lack any junctions.

A nitride-oxide layer insulator is used in the MNOS (metal-nitride-oxide-semiconductor transistor) between the gate and the body.

Ion concentrations in a solution can be measured using an ISFET (ion-sensitive field-effect transistor); if the ion concentration (such as H+, see pH electrode) varies, the current through the transistor will also change.

When a charged molecule is present, changes in the electrostatic field at the BioFET surface lead to a measurably different change in current through the transistor. The BioFET (Biologically sensitive field-effect transistor) is a class of sensors/biosensors based on ISFET technology. These include FETs that have been altered by an enzyme (EnFETs), an immune system (ImmunoFETs), a gene (GenFETs), DNAFETs, a cell-based bioFET (CPFETs), a beetle/chip FET (BeetleFETs), and FETs based on ion channels or protein binding.

A specialised FET called a DNAFET (DNA field-effect transistor) functions as a biosensor by using a gate built of single-strand DNA molecules to find compatible DNA strands.

On high density processor chips, finFET, including GAAFET or a gate-all-around FET, is employed.

A biosensor and chemical sensor, the GFET is an extremely sensitive graphene-based field effect transistor. Due to graphene's two-dimensional structure and physical characteristics, GFETs provide greater sensitivity and fewer "false positives" in sensing applications. Such devices may have application as non-volatile memory because the Fe FET uses a ferroelectric between the gate, allowing the transistor to retain its state in the absence of bias.

IBM's 2021 finFET upgrade, the VTFET, or Vertical-Transport Field-Effect Transistor, allows for more density and lower power.

FET Advantages

Field-effect transistors offer a high degree of isolation between control and flow thanks to their high gate-to-drain current resistance, which is on the order of 100 M or more. A FET typically produces less noise than a bipolar junction transistor (BJT), and is used in noise-sensitive electronics like tuners and low-noise amplifiers for VHF and satellite receivers because base current noise will grow with shaping time[clarification needed]. It is comparatively radiation resistant. At zero drain current, it displays no offset voltage and works great as a signal chopper. Compared to a BJT, it often offers higher thermal stability.

When the gate is closed or opened, there is no additional power demand as there would be with a bipolar junction transistor or with non-latching relays in some states because the FETs are controlled by gate charge. Due to the incredibly low power switching made possible by this, circuits can be made smaller than with other types of switches because less heat needs to be dissipated.

FET Disadvantages

When compared to a bipolar junction transistor, the gain-bandwidth product of a field-effect transistor is comparatively low. Because MOSFETs are extremely vulnerable to overload voltages, installation calls for great care. The MOSFET is susceptible to electrostatic discharge and fluctuations in threshold voltage due to the thin insulating layer that separates the gate from the channel. Once the gadget has been fitted in a circuit that has been appropriately built, this is typically not a problem.

FET Failure Modes

When used within the manufacturer's specified temperature and electrical limits, field-effect transistors are comparatively durable (proper derating). However, a body diode is frequently included in contemporary FET devices. The FET may exhibit slow body diode behaviour, where a parasitic transistor will activate and let high current to be carried from the drain to the source when the FET is off, if the body diode's properties are not taken into account.

FET Uses

The MOSFET is the FET that is most frequently utilised. For contemporary digital integrated circuits, the CMOS (complementary metal oxide semiconductor) manufacturing technology serves as the foundation. In this process technique, the p-channel and n-channel MOSFETs are coupled in series so that when one is on, the other is off (this arrangement is typically referred to as "enhancement-mode").

When used in the linear mode, FETs allow electrons to move across the channel in any direction. The devices are often (but not always) designed symmetrically from source to drain, hence the names drain terminal and source terminal are somewhat arbitrary. FETs can thus be used to switch analogue signals across pathways (multiplexing). This idea can be used, for instance, to build a solid-state mixing board. FETs are frequently utilised as amplifiers. For instance, in a common-drain (source follower) configuration, it works well as a buffer due to its high input resistance and low output resistance.

When switching internal combustion engine ignition coils, IGBTs are used because of their quick switching and voltage blocking characteristics.

Source-gated transistor

In large-area electronics like display screens, source-gated transistors are more resistant to manufacturing and environmental problems but operate more slowly than FETs.

2N3819 Transistor

2N3819 is a TO-92 packaged JFET transistor built for VHF and UHF applications. The transistor possesses some very good features like very low noise and distortion, high gain and sensitivity, quality signal amplification and can also be used for very low level signal amplification. Other than that it can also be used as a switch. When used as a switch it is capable of fast switching and can drive loads that falls under 10mA. It also provides high gain at 100MHz.


Designed for low voltage, high speed switching applications in power supplies.

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