Metal gate

gatemetal-gatealuminum-gategate metalsgatesinsulated-gatemetal gates
A metal gate, in the context of a lateral metal-oxide-semiconductor (MOS) stack, is just that—the gate material is made from a metal.wikipedia
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MOSFET

metal-oxide-semiconductorMOSMOS integrated circuit
A metal gate, in the context of a lateral metal-oxide-semiconductor (MOS) stack, is just that—the gate material is made from a metal. The first MOSFET (metal-oxide-semiconductor field-effect transistor, or MOS transistor) was invented by Egyptian engineer Mohamed Atalla and Korean engineer Dawon Kahng at Bell Labs in 1959, and demonstrated in 1960.
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET), also known as the metal–oxide–silicon transistor (MOS transistor, or MOS), is a type of insulated-gate field-effect transistor (IGFET) that is fabricated by the controlled oxidation of a semiconductor, typically silicon.

Mohamed M. Atalla

Mohamed AtallaMartin Mohamed AtallaMohamed Mohamed Atalla
The first MOSFET (metal-oxide-semiconductor field-effect transistor, or MOS transistor) was invented by Egyptian engineer Mohamed Atalla and Korean engineer Dawon Kahng at Bell Labs in 1959, and demonstrated in 1960.
In 1960, Atalla and Kahng fabricated the first MOSFET with a gate oxide thickness of 100 nm, along with a gate length of 20µm.

CMOS

RF CMOScomplementary metal–oxide–semiconductorcomplementary MOS
In NMOS and CMOS technologies, over time and elevated temperatures, the positive voltages employed by the gate structure can cause any existing positively charged sodium impurities directly under the positively charged gate to diffuse through the gate dielectric and migrate to the less-positively-charged channel surface, where the positive sodium charge has a higher effect on the channel creation – thus lowering the threshold voltage of an N-channel transistor and potentially causing failures over time.
The phrase "metal–oxide–semiconductor" is a reference to the physical structure of MOS field-effect transistors, having a metal gate electrode placed on top of an oxide insulator, which in turn is on top of a semiconductor material.

Self-aligned gate

silicon-gatesilicon gateself-aligned
Polysilicon is also attractive for the easy manufacturing of self-aligned gates.
Prior to these innovations, self-aligned gates had been demonstrated on metal-gate devices, but their real impact was on silicon-gate devices.

Gate oxide

oxidegate dielectriccovered gate
The gate oxide is the dielectric layer that separates the gate terminal of a MOSFET (metal-oxide-semiconductor field-effect transistor) from the underlying source and drain terminals as well as the conductive channel that connects source and drain when the transistor is turned on.

Multigate device

GAAFETgate-all-arounddouble-gate
Sekigawa fabricated an XMOS device with 2µm gate length in 1987.

Transistor

transistorstransistorizedsilicon transistor
However, polysilicon doped at practical levels does not offer the near-zero electrical resistance of metals, and is therefore not ideal for charging and discharging the gate capacitance of the transistor – resulting in slower circuitry.
It has an insulated gate, whose voltage determines the conductivity of the device.

Dawon Kahng

The first MOSFET (metal-oxide-semiconductor field-effect transistor, or MOS transistor) was invented by Egyptian engineer Mohamed Atalla and Korean engineer Dawon Kahng at Bell Labs in 1959, and demonstrated in 1960.

Bell Labs

Bell LaboratoriesBell Telephone LaboratoriesAT&T Bell Laboratories
The first MOSFET (metal-oxide-semiconductor field-effect transistor, or MOS transistor) was invented by Egyptian engineer Mohamed Atalla and Korean engineer Dawon Kahng at Bell Labs in 1959, and demonstrated in 1960.

Silicon

Sisilicon revolutionsilicium
They used silicon as channel material and a non-self-aligned aluminium (Al) gate. A material called polysilicon (polycrystalline silicon, highly doped with donors or acceptors to reduce its electrical resistance) was used to replace aluminum.

Aluminium

aluminumAlall-metal
They used silicon as channel material and a non-self-aligned aluminium (Al) gate. A material called polysilicon (polycrystalline silicon, highly doped with donors or acceptors to reduce its electrical resistance) was used to replace aluminum. Particularly, metal (most commonly aluminum – a Type III (P-type) dopant) has a tendency to disperse into (alloy with) silicon during these thermal annealing steps.

Polycrystalline silicon

polysiliconmulti-crystalline siliconpolycrystalline
Polysilicon is also attractive for the easy manufacturing of self-aligned gates. However, polysilicon doped at practical levels does not offer the near-zero electrical resistance of metals, and is therefore not ideal for charging and discharging the gate capacitance of the transistor – resulting in slower circuitry. A material called polysilicon (polycrystalline silicon, highly doped with donors or acceptors to reduce its electrical resistance) was used to replace aluminum. Polysilicon can be deposited easily via chemical vapor deposition (CVD) and is tolerant to subsequent manufacturing steps which involve extremely high temperatures (in excess of 900–1000 °C), where metal was not. Polysilicon gates – while sensitive to the same phenomenon, could be exposed to small amounts of HCl gas during subsequent high-temperature processing (commonly called "gettering") to react with any sodium, binding with it to form NaCl and carrying it away in the gas stream, leaving an essentially sodium-free gate structure – greatly enhancing reliability.

Crystallite

polycrystallinegraingrains
A material called polysilicon (polycrystalline silicon, highly doped with donors or acceptors to reduce its electrical resistance) was used to replace aluminum.

Doping (semiconductor)

dopingdopeddope
A material called polysilicon (polycrystalline silicon, highly doped with donors or acceptors to reduce its electrical resistance) was used to replace aluminum.

Chemical vapor deposition

chemical vapour depositionCVDLPCVD
Polysilicon can be deposited easily via chemical vapor deposition (CVD) and is tolerant to subsequent manufacturing steps which involve extremely high temperatures (in excess of 900–1000 °C), where metal was not.

Alloy

alloysmetal alloyalloying
Particularly, metal (most commonly aluminum – a Type III (P-type) dopant) has a tendency to disperse into (alloy with) silicon during these thermal annealing steps.

Rapid thermal processing

Rapid thermal annealannealing processesRTP
Particularly, metal (most commonly aluminum – a Type III (P-type) dopant) has a tendency to disperse into (alloy with) silicon during these thermal annealing steps.

Wafer (electronics)

wafersilicon waferwafers
In particular, when used on a silicon wafer with a crystal orientation, excessive alloying of aluminum (from extended high temperature processing steps) with the underlying silicon can create a short circuit between the diffused FET source or drain areas under the aluminum and across the metallurgical junction into the underlying substrate – causing irreparable circuit failures.

Short circuit

short-circuitshortelectrical short
In particular, when used on a silicon wafer with a crystal orientation, excessive alloying of aluminum (from extended high temperature processing steps) with the underlying silicon can create a short circuit between the diffused FET source or drain areas under the aluminum and across the metallurgical junction into the underlying substrate – causing irreparable circuit failures.

Lithography

lithographlithographerlithographs
The implantation or diffusion of source and drain dopant impurities is carried out with the gate in place, leading to a channel perfectly aligned to the gate without additional lithographic steps with the potential for misalignment of the layers.

Sodium

NaNa + sodium ion
In NMOS and CMOS technologies, over time and elevated temperatures, the positive voltages employed by the gate structure can cause any existing positively charged sodium impurities directly under the positively charged gate to diffuse through the gate dielectric and migrate to the less-positively-charged channel surface, where the positive sodium charge has a higher effect on the channel creation – thus lowering the threshold voltage of an N-channel transistor and potentially causing failures over time. Polysilicon gates – while sensitive to the same phenomenon, could be exposed to small amounts of HCl gas during subsequent high-temperature processing (commonly called "gettering") to react with any sodium, binding with it to form NaCl and carrying it away in the gas stream, leaving an essentially sodium-free gate structure – greatly enhancing reliability.

Threshold voltage

gate voltagelow threshold voltagepinch-off voltage
In NMOS and CMOS technologies, over time and elevated temperatures, the positive voltages employed by the gate structure can cause any existing positively charged sodium impurities directly under the positively charged gate to diffuse through the gate dielectric and migrate to the less-positively-charged channel surface, where the positive sodium charge has a higher effect on the channel creation – thus lowering the threshold voltage of an N-channel transistor and potentially causing failures over time.

Hydrogen chloride

HClanhydrous hydrochloric acidHCl gas
Polysilicon gates – while sensitive to the same phenomenon, could be exposed to small amounts of HCl gas during subsequent high-temperature processing (commonly called "gettering") to react with any sodium, binding with it to form NaCl and carrying it away in the gas stream, leaving an essentially sodium-free gate structure – greatly enhancing reliability.

Getter

getteringBarium flash getterNEG
Polysilicon gates – while sensitive to the same phenomenon, could be exposed to small amounts of HCl gas during subsequent high-temperature processing (commonly called "gettering") to react with any sodium, binding with it to form NaCl and carrying it away in the gas stream, leaving an essentially sodium-free gate structure – greatly enhancing reliability.

Electrical resistance and conductance

resistanceelectrical resistanceconductance
However, polysilicon doped at practical levels does not offer the near-zero electrical resistance of metals, and is therefore not ideal for charging and discharging the gate capacitance of the transistor – resulting in slower circuitry.