![FET comparison](https://upload.wikimedia.org/wikipedia/commons/c/c0/FET_comparison.png)
In
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 contro ...
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 Zincblende (crystal structure), zinc blende crystal structure.
Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monoli ...
and
germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbors s ...
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) c ...
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 V
DD supply and operated in the saturation region (sometimes the gates are biased to a higher V
GG voltage and operated in the linear region, for a better
power–delay product
In digital electronics, the power–delay product (PDP) is a figure of merit correlated with the energy efficiency (electricity), energy efficiency of a logic gate or logic family. Also known as switching energy, it is the product of power consumpt ...
(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 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 log ...
, and the 1976
Zilog Z80 used depletion-load silicon-gate NMOS.
History
The first
MOSFET
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
(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 tab ...
semiconductor device
A semiconductor device is an electronic component that relies on the electronic properties of a semiconductor material (primarily silicon, germanium, and gallium arsenide, as well as organic semiconductors) for its function. Its conductivity li ...
. In 1963, both depletion- and enhancement-mode MOSFETs were described by Steve R. Hofstein and Fred P. Heiman at
RCA Laboratories. In 1966, T. P. Brody and H. E. Kunig at
Westinghouse Electric 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 galli ...
(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