The field-effect transistor (FET) is a type of transistor that relies on an electric field to control the shape and hence the conductivity of a 'channel' in a semiconductor material. In Electronics, a transistor is a Semiconductor device commonly used to amplify or switch electronic signals In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can Electrical conductivity or specific conductivity is a measure of a material's ability to conduct an Electric current. A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that The concept of the field effect transistor predates the bipolar junction transistor (BJT), though it was not physically implemented until after BJTs, due to the limitations of semiconductor materials and relative ease of manufacturing BJTs compared to FETs at the time. A bipolar (junction transistor ( BJT) is a type of Transistor.
All FETs except J-FETs have four terminals, which are known as the gate, drain, source and body/base/bulk/substrate. Compare these to the terms used for BJTs: base, collector and emitter. BJTs and J-FETs have no body terminal.
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. Electrons flow from the source terminal towards the drain terminal if 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 type. The body terminal and the source terminal are sometimes connected together since the source is also sometimes connected to the highest or lowest voltage within the circuit, however there are several uses of FETs which do not have such a configuration, such as transmission gates and cascode circuits. A transmission gate is an electronic element It is a good non-mechanical Relay, built with CMOS technology The cascode is a two-stage Amplifier composed of a Transconductance amplifier followed by a current buffer.
The FET can be constructed from a number of semiconductors, silicon being by far the most common. Silicon (ˈsɪlɪkən or /ˈsɪlɪkɒn/ silicium is the Chemical element that has the symbol Si and Atomic number 14 Most FETs are made with conventional bulk semiconductor processing techniques, using the single crystal semiconductor wafer as the active region, or channel. Semiconductor device fabrication is the process used to create chips the Integrated circuits that are present in everyday Electrical and electronic A single crystal, also called monocrystal, is a Crystalline Solid in which the Crystal lattice of the entire sample is continuous and unbroken A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that
Among the more unusual body materials are amorphous silicon, polycrystalline silicon or other amorphous semiconductors in thin-film transistors or organic field effect transistors that are based on organic semiconductors and often apply organic gate insulators and electrodes. Amorphous silicon (a-Si is the non-crystalline Allotropic form of Silicon. An Organic Field-Effect Transistor (OFET is a Field effect transistor using an Organic semiconductor in its channel An organic semiconductor is any Organic material that has Semiconductor properties
The channel of a FET (explained below) is doped to produce either an N-type semiconductor or a P-type semiconductor. In Semiconductor production doping is the process of intentionally introducing impurities into an extremely pure (also referred to as intrinsic) semiconductor to An N-type semiconductor (N for Negative) is obtained by carrying out a process of doping, that is by adding an impurity of valence -five elements to A P-type semiconductor (P for Positive) is obtained by carrying out a process of doping, that is adding a certain type of atoms to the semiconductor in order 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 are:
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. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J An electron hole is the conceptual and mathematical Opposite of an Electron, useful in the study of Physics and Chemistry. (For ease of discussion, this assumes body and source are connected). This conductive channel is the "stream" through which electrons flow from source to drain.
Consider 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. In Semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region If the depletion 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. Likewise a positive gate-to-source voltage increases the channel size and allows electrons to flow easily.
Now consider an n-channel "enhancement-mode" device. A positive gate-to-source voltage is necessary to create a conductive channel, since one does not exist naturally within the transistor. 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 free of mobile carriers called a depletion region, and the phenomenon is referred to as the threshold voltage of the FET. In Semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region The threshold voltage of a MOSFET is usually defined as the gate voltage where an inversion layer forms at the interface between the insulating layer (oxide and the substrate Further gate-to-source voltage increase will attract even more electrons towards the gate which are able to create a conductive channel from source to drain; this process is called inversion.
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; 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. In Physics, a charge carrier denotes a free (mobile unbound particle carrying an Electric charge. Considering again an n-channel device, a depletion region exists in the p-type body, surrounding the conductive channel and drain and source regions. In Semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region 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. Silicon (ˈsɪlɪkən or /ˈsɪlɪkɒn/ silicium is the Chemical element that has the symbol Si and Atomic number 14 Any increase of the drain-to-source voltage will increase the distance from drain to the pinch-off point, increasing resistance due to the depletion region proportionally to the applied drain-to-source voltage. This proportional change causes the drain-to-source current to remain relatively fixed independent of changes to the drain-to-source voltage and quite unlike the linear mode operation. Thus in saturation mode, the FET behaves as a constant-current source rather than as a resistor and can be used most effectively as a voltage amplifier. A current source is an electrical or electronic device that delivers or absorbs electric current In this case, the gate-to-source voltage determines the level of constant current through the channel.
The most commonly used FET is the MOSFET. The metal–oxide–semiconductor field-effect transistor ( MOSFET, MOS-FET, or MOS FET) is a device used to amplify or switch electronic signals The CMOS (complementary-symmetry metal oxide semiconductor) process technology is the basis for modern digital integrated circuits. Complementary metal–oxide–semiconductor ( CMOS) (pronounced "see-moss" siːmɔːs ˈsiːmɒs is a major class of Integrated circuits CMOS technology A digital system uses discrete (discontinuous values usually but not always Symbolized Numerically (hence called "digital" to represent information for Microchipsjpg|right|thumb|200px|Microchips ( EPROM memory with a transparent window showing the integrated circuit inside 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 the Microelectronics industry a semiconductor fabrication plant (commonly called a fab) is a factory where devices such as Integrated circuits are manufactured
The fragile insulating layer of the MOSFET between the gate and channel makes it vulnerable to electrostatic damage during handling. Electrostatic discharge ( ESD) is the sudden and momentary Electric current that flows between two objects at different Electrical potentials The term is This is not usually a problem after the device has been installed.
In FETs electrons can flow in either direction through the channel when operated in the linear mode, and the naming convention of drain terminal and source terminal is somewhat arbitrary, as the devices are typically (but not always) built symmetrically from source to drain. This makes FETs suitable for switching analog signals between paths (multiplexing). For multiplexing in electronics and signal processing see Multiplexer. With this concept, one can construct a solid-state mixing board, for example. In professional audio, a mixing console, or audio mixer, also called a sound board or soundboard, is an electronic device for combining