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field-effect transistor The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs ( JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs co ...
s (FETs), depletion mode and enhancement mode are two major transistor types, corresponding to whether the transistor is in an on state or an off state at zero gate–source voltage. Enhancement-mode MOSFETs (metal–oxide–semiconductor FETs) are the common switching elements in most integrated circuits. These devices are off at zero gate–source voltage. NMOS can be turned on by pulling the gate voltage higher than the source voltage, PMOS can be turned on by pulling the gate voltage lower than the source voltage. In most circuits, this means pulling an enhancement-mode MOSFET's gate voltage towards its drain voltage turns it on. In a depletion-mode MOSFET, the device is normally on at zero gate–source voltage. Such devices are used as load "resistors" in logic circuits (in depletion-load NMOS logic, for example). For N-type depletion-load devices, the threshold voltage might be about −3 V, so it could be turned off by pulling the gate 3 V negative (the drain, by comparison, is more positive than the source in NMOS). In PMOS, the polarities are reversed. The mode can be determined by the sign of the threshold voltage (gate voltage relative to source voltage at the point where an inversion layer just forms in the channel): for an N-type FET, enhancement-mode devices have positive thresholds, and depletion-mode devices have negative thresholds; for a P-type FET, enhancement-mode have negative, and depletion-mode have positive. Junction field-effect transistors (
JFET The junction-gate field-effect transistor (JFET) is one of the simplest types of field-effect transistor. JFETs are three-terminal semiconductor devices that can be used as electronically controlled switches or resistors, or to build amplifiers. ...
s) are depletion-mode, since the gate junction would forward bias if the gate were taken more than a little from source toward drain voltage. Such devices are used in
gallium arsenide Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a zinc blende crystal structure. Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monolithic microwave integrated circ ...
and germanium chips, where it is difficult to make an oxide insulator.


Alternative terminology

Some sources say "depletion type" and "enhancement type" for the device types as described in this article as "depletion mode" and "enhancement mode", and apply the "mode" terms for which direction the gate–source voltage differs from zero. Moving the gate voltage toward the drain voltage "enhances" the conduction in the channel, so this defines the enhancement mode of operation, while moving the gate away from the drain depletes the channel, so this defines depletion mode.


Enhancement-load and depletion-load logic families

Depletion-load NMOS logic refers to the logic family that became dominant in silicon
VLSI Very large-scale integration (VLSI) is the process of creating an integrated circuit (IC) by combining millions or billions of MOS transistors onto a single chip. VLSI began in the 1970s when MOS integrated circuit (Metal Oxide Semiconductor) ...
in the latter half of the 1970s; the process supported both enhancement-mode and depletion-mode transistors, and typical logic circuits used enhancement-mode devices as pull-down switches and depletion-mode devices as loads, or pull-ups. Logic families built in older processes that did not support depletion-mode transistors were retrospectively referred to as ''enhancement-load'' logic, or as ''saturated-load'' logic, since the enhancement-mode transistors were typically connected with gate to the VDD supply and operated in the saturation region (sometimes the gates are biased to a higher VGG voltage and operated in the linear region, for a better power–delay product (PDP), but the loads then take more area). Alternatively, rather than static logic gates, dynamic logic such as four-phase logic was sometimes used in processes that did not have depletion-mode transistors available. For example, the 1971
Intel 4004 The Intel 4004 is a 4-bit central processing unit (CPU) released by Intel Corporation in 1971. Sold for US$60, it was the first commercially produced microprocessor, and the first in a long line of Intel CPUs. The 4004 was the first signific ...
used enhancement-load silicon-gate
PMOS logic PMOS or pMOS logic (from p-channel metal–oxide–semiconductor) is a family of digital circuits based on p-channel, enhancement mode metal–oxide–semiconductor field-effect transistors (MOSFETs). In the late 1960s and early 1970s, PMOS lo ...
, and the 1976 Zilog Z80 used depletion-load silicon-gate NMOS.


History

The first MOSFET (metal-oxide-semiconductor field-effect transistor) demonstrated by Egyptian engineer Mohamed M. Atalla and Korean engineer Dawon Kahng at
Bell Labs Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984), then AT&T Bell Laboratories (1984–1996) and Bell Labs Innovations (1996–2007), is an American industrial research and scientific development company owned by mult ...
in 1960 was an enhancement-mode
silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...
semiconductor device. In 1963, both depletion- and enhancement-mode MOSFETs were described by Steve R. Hofstein and Fred P. Heiman at
RCA Laboratories The RCA Corporation was a major American electronics company, which was founded as the Radio Corporation of America in 1919. It was initially a patent trust owned by General Electric (GE), Westinghouse, AT&T Corporation and United Fruit Com ...
. In 1966, T. P. Brody and H. E. Kunig at
Westinghouse Electric The Westinghouse Electric Corporation was an American manufacturing company founded in 1886 by George Westinghouse. It was originally named "Westinghouse Electric & Manufacturing Company" and was renamed "Westinghouse Electric Corporation" in ...
fabricated enhancement- and depletion-mode
indium arsenide Indium arsenide, InAs, or indium monoarsenide, is a narrow-bandgap semiconductor composed of indium and arsenic. It has the appearance of grey cubic crystals with a melting point of 942 °C. Indium arsenide is similar in properties to galliu ...
(InAs) MOS thin-film transistors (TFTs). In 2022, the first dual-mode organic transistor that behaves in both depletion mode and enhancement mode was reported by a team at University of California-Santa Barbara.


References

{{reflist Transistor types MOSFETs