The Field-Effect Transistor (FET) is a type of transistor that works by modulating an electric field inside a semiconductor material. The most common types of FETs are MOSFETs, JFETs, MESFETs, HEMTs, and TFTs.
Most FETs are made using conventional bulk semiconductor processing techniques, which use a single crystal of silicon as the starting material. TFTs (Thin-film transistors) are made by deposited thin-films, by some CVD process.
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The MOSFET, or Metal-Oxide-Semiconductor Field-Effect Transistor, is made up of channel of n-type of p-type doped semiconductor material. The channel is connected on each end to source and drain terminals which are oppositely doped in relation to the channel. The gate terminal is a layer of "polysilicon" (polycrystalline silicon; earlier models used aluminum instead) placed over the channel, but separated from the channel by a thin layer of insulating silicon dioxide. When a voltage is applied between the gate and source terminals, the electric field generated penetrates through the oxide and creates a so-called "inversion channel" in the channel underneath. Varying the voltage between gate and body modulates the thickness of this conductive layer and so makes it possible to control the current flow between drain and source.
The schematic symbols for p- and n-channel MOSFETs. The symbols to the right include an extra terminal for the transistor body whereas in those to the left the body is implicitly connected to the source.
MOSFETs are ideal for switching, especially in digital circuits and switched-mode power supplies. Their low on-resistance also makes them suitable replacements for diodes (so-called OR-ing diodes) used to connect the outputs of power supplies in parallel. The growth of digital technologies like the microprocessors have provided the motivation to advance MOSFET technology faster than any other type of silicon-based transistor. One advantage of MOSFETs for digital switching is that the oxide layer between the gate and the channel prevents any DC current from flowing, making design easier and reducing power consumption. As switching speeds increase, however, large quantities of current are consumed by the charging and discharging of gate capacitance, erasing any power savings from the high input resistance. MOSFETs also have a problem with static discharge: the thin layer of glass is very fragile and can be penetrated by low voltages. The maximum voltage that can be safely sustained across this insulating silicon-dioxide gate is dependent on its thickness: an older MOSFET with an oxide thickness of 0.1Ám (or 1000angstroms) can handle 30volts while a modern (2002) MOSFET with a glass thickness of nearer 3nm (or 30angstroms) might only manage a volt safely. Manufacturers normally boast about the channel length of the MOSFET rather than the oxide thickness but there is a strong relationship: the length is always around 50 times greater than the oxide thickness, thus the above MOSFET with 3nm thickness would correspond to a manufacturer's claim of a 0.13Ám MOSFET length.
Cross section of n-channel MOSFET as found in integrated circuits
There are two types of MOSFETs, depending on the type of doping: n-channel MOSFETs have n-doped source and drains and are p-doped under the gate, while p-channel MOSFETs are reversed. The difference is important since applying a positive voltage (relative to the source) to an n-channel MOSFET's gate will make it conductive, while applying the same voltage to a p-channel MOSFET will make it non-conductive.
There are also depletion mode MOSFET devices, which are less commonly used than the standard "enhancement mode" devices already described. These are MOSFET devices which are doped so that a channel exists even without any voltage applied to the gate. When one then applies a voltage to the gate, the channel is depleted, which reduces the current flow through the device. In essence the depletion mode device is equivalent to a normally closed switch, while the enhancement mode device is equivalent to a normally open switch.
Historically, n-channel MOSFETs tended to be smaller and therefore cheaper to produce. These were the driving principles in the design of NMOS logic which uses n-channel MOSFETs exclusively. However, NMOS logic consumes power even when no switching is taking place, unlike CMOS logic which combines n-channel and p-channel MOSFETs on a single chip. With advances in technology, CMOS logic displaced NMOS in the 1980s to become the preferred choice for digital chips.
The simplest type of FET is the JFET, or Junction Field-Effect Transistor. It
consists of a long channel of semiconductor material, either P or N doped, with a contact on each end, labeled source and drain respectively. The third control terminal, called the gate, is placed to contact the edges of the channel, and is doped opposite to the polarity of the channel. When a voltage is applied between source and drain, current flows. The current flow can be modulated by applying a voltage between the gate and source terminals. When this occurs, the electric field applied effectively narrows the channel, and the flow of current is restricted.
JFETs have several advantages over the historically important BJT. They do not require any input current to function, which makes them useful for circuits requiring a high input impedance. However, their gain is usually relatively low in comparison. They are used in low-noise, low-signal level analog applications, and sometimes used in switching applications.
MESFET stands for MEtal-Semiconductor Field Effect Transistor. It is quite similar to a JFET in construction and terminology. The difference is that instead of a using a p-n junction for a gate, a Schottky (metal-semiconductor) junction is used. MESFETs are usually constructed in GaAs or InP (never silicon), and hence are faster but more expensive than silicon-based JFETs or MOSFETs. MESFETs are used up to approximately 30GHz, but building a computer processor using them will probably not be economic for some time. MESFETs are commonly used for microwave frequency communications and radar.
HEMT stands for High Electron Mobility Transistor. A HEMT is a MESFET with a junction between two materials with different band gaps (i.e. a heterojunction) as the channel instead of an n-doped region. A commonly used combination is GaAs with AlGaAs. The effect of this junction is to create a very thin layer where the Fermi energy is above the conduction band, giving the channel very low resistance (or to put it another way, "high electron mobility"). This layer is sometimes calles a two-dimensional electron gas. As with all the other types of FETs, a voltage applied to the gate alters the conductivity of this layer.
Ordinarily, the two different materials used for a heterojunction must have the same lattice constant (spacing between the atoms). An analogy - imagine pushing together two plastic combs with a slightly different spacing - at regular intervals, you'll see two teeth clump together. In semiconductors, these discontinuities are a kind of "trap", and greatly reduce device performance.
A HEMT where this rule is violated is called a PHEMT or pseudomorphic HEMT. This feat is achieved by using an extremely thin layer of one of the materials - so thin that it simply stretches to fit the other material. This technique allows the construction of transistors with bigger bandgap differences than otherwise possible. This gives them better performance.
To the best of the author's knowledge, PHEMTs and related devices are the fastest transistors available. They can be used to make amplifiers which work at over 200 GHz. Applications are similar to MESFETs - microwave and millimetre wave communications, radar, and radio astronomy.
Cross section of an InGaAs PHEMT