field-effect transistor The field-effect transistor (FET) is a type of transistor that uses an electric field to control the current through a semiconductor. It comes in two types: junction FET (JFET) and metal-oxide-semiconductor FET (MOSFET). FETs have three termi ...

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 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. U ...

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; it has Symbol (chemistry), symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid or a nonmetal in the carbon group that is chemically ...

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 In integrated circuits, depletion-load NMOS is a form of digital logic family that uses only a single power supply voltage, unlike earlier NMOS logic, NMOS (n-type metal-oxide semiconductor) logic families that needed multiple power supply voltag ...

refers to the logic family that became dominant in silicon VLSI 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 (PDP), but the loads then take more area). Alternatively, rather than static logic gates, dynamic logic such as

four-phase logic Four-phase logic is a type of, and design methodology for dynamic logic. It enabled non-specialist engineers to design quite complex ICs, using either PMOS or NMOS processes. It uses a kind of 4-phase clock signal to smooth out power delivery ...

was sometimes used in processes that did not have depletion-mode transistors available. For example, the 1971

Intel 4004 The Intel 4004 was part of the 4 chip MCS-4 micro computer set, released by the Intel, Intel Corporation in November 1971; the 4004 being part of the first commercially marketed microprocessor chipset, and the first in a long line of List of I ...

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 ...

, and the 1976

Zilog Z80 The Zilog Z80 is an 8-bit computing, 8-bit microprocessor designed by Zilog that played an important role in the evolution of early personal computing. Launched in 1976, it was designed to be Backward compatibility, software-compatible with the ...

used depletion-load silicon-gate NMOS.

History

The original two types of MOSFET logic gates, PMOS and NMOS, were developed by Frosch and Derick in 1957 at Bell Labs. In 1963, both depletion- and enhancement-mode MOSFETs were described by Steve R. Hofstein and Fred P. Heiman at

RCA Laboratories RCA Corporation was a major American electronics company, which was founded in 1919 as the Radio Corporation of America. It was initially a patent trust owned by General Electric (GE), Westinghouse, AT&T Corporation and United Fruit Company ...

. 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 and headquartered in Pittsburgh, Pennsylvania. It was originally named "Westinghouse Electric & Manufacturing Company" and was ...

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 gallium ars ...

(InAs) MOS

thin-film transistors A thin-film transistor (TFT) is a special type of field-effect transistor (FET) where the transistor is made by thin film deposition. TFTs are grown on a supporting (but non-conducting) substrate, such as glass. This differs from the convention ...

(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