FIELD EFFECT TRANSISTOR

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 0.1µm (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 0.13µm 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.

MOSFETs can only be constructed in silicon, not GaAs or InP, due to the lack of a suitable insulator to put under the gate.

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.

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