The field-effect transistor (FET) is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with the dual-carrier-type operation of bipolar (junction) transistors (BJT).
The field effect transistor was actually conceived before the more familiar bipolar transistor. Due to limited technology and later the rapid rise of the bipolar device it was not pursued until the early 1960Õs as a viable semiconductor alternative. At this time further investigation of the field effect transistor and advances in semiconductor process technology lead to the types in use today. Field effect transistors include the Junction FET (JFET) and the MOSFET. The MOSFET is a metal oxide semiconductor technology and is sometimes referred to as the IGFET or Insulated Gate FET. All field effect transistors are majority carrier devices. This means that current is conducted by the majority carrier species present in the channel of the FET. This majority carrier consists of hole for p-channel devices and electrons for n-channel devices. The JFET operates with current flow through a controlled channel in the semiconductor material. The MOSFET creates a channel under the insulated gate region which is produced by an electric field induced in the semiconductor by applying a voltage to the gate. The JFET is a depletion mode device whereas the MOSFET can operate as a depletion mode or an enhancement mode device. Depletion mode devices are controlled by depleting the current channel of charge carriers. Enhancement mode devices are controlled by enhancing the channel with additional charge carriers.
The FET's three terminals are...
Source (S), through which the majority carriers enter the channel. Conventional current entering the channel at S is designated by IS.
Drain (D), through which the majority carriers leave the channel. Conventional current entering the channel at D is designated by ID. Drain to Source voltage is VDS.
Gate (G), the terminal that modulates the channel conductivity. By applying voltage to G, one can control ID.
Types of field-effect transistors
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 depletion mode FETs, or doped of similar type to the channel as in enhancement mode FETs. Field-effect transistors are also distinguished by the method of insulation between channel and gate. Types of FETs are:
The MOSFET (Metal–Oxide–Semiconductor Field-Effect Transistor) utilizes an insulator (typically SiO2) between the gate and the body.
The CNTFET (Carbon nanotube field-effect transistor)
The DEPFET is a FET formed in a fully depleted substrate and acts as a sensor, amplifier and memory node at the same time. It can be used as an image (photon) sensor.
The DGMOSFET is a MOSFET with dual gates.
The DNAFET is a specialized FET that acts as a biosensor, by using a gate made of single-strand DNA molecules to detect matching DNA strands.
The FREDFET (Fast Reverse or Fast Recovery Epitaxial Diode FET) is a specialized FET designed to provide a very fast recovery (turn-off) of the body diode.
The HEMT (high electron mobility transistor), also called a HFET (heterostructure FET), can be made using bandgap engineering in a ternary semiconductor such as AlGaAs. The fully depleted wide-band-gap material forms the isolation between gate and body.
The IGBT (insulated-gate bipolar transistor) is a device for power control. It has a structure akin to a MOSFET coupled with a bipolar-like main conduction channel. These are commonly used for the 200-3000 V drain-to-source voltage range of operation. Power MOSFETs are still the device of choice for drain-to-source voltages of 1 to 200 V.
The ISFET (ion-sensitive field-effect transistor) used to measure ion concentrations in a solution; when the ion concentration (such as H+, see pH electrode) changes, the current through the transistor will change accordingly.
The JFET (junction field-effect transistor) uses a reverse biased p-n junction to separate the gate from the body.
The MESFET (Metal–Semiconductor Field-Effect Transistor) substitutes the p-n junction of the JFET with a Schottky barrier; used in GaAs and other III-V semiconductor materials.
The MODFET (Modulation-Doped Field Effect Transistor) uses a quantum well structure formed by graded doping of the active region.
The NOMFET is a Nanoparticle Organic Memory Field-Effect Transistor.
The OFET is an Organic Field-Effect Transistor using an organic semiconductor in its channel.
The GNRFET is a Field-Effect Transistor that uses a graphene nanoribbon for its channel.
The VeSFET (Vertical-Slit Field-Effect Transistor) is a square-shaped junction-less FET with a narrow slit connecting the source and drain at opposite corners. Two gates occupy the other corners, and control the current through the slit
The ChemFET, or chemical field-effect transistor, is a type of a field-effect transistor acting as a chemical sensor. It is a structural analog of a MOSFET transistor, where the charge on the gate electrode is applied by a chemical process. It may be used to detect atoms, molecules, and ions in liquids and gases
advantage of FET
The main advantage of the FET is its high input resistance, on the order of 100M ohms or more. Thus, it is a voltage-controlled device, and shows a high degree of isolation between input and output. It is a unipolar device, depending only upon majority current flow. It is less noisy and is thus found in FM tuners for quiet reception. It is relatively immune to radiation. It exhibits no offset voltage at zero drain current and hence makes an excellent signal chopper. It typically has better thermal stability than a BJT.
Disadvantages of FET
It has relatively low gain-bandwidth product compared to a BJT. The MOSFET has a drawback of being very susceptible to overload voltages, thus requiring special handling during installation.
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