Metal gate

[citation needed] 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.

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.

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 – potentially resulting in slower circuitry.

From the 45 nm node onward, the metal gate technology returns, together with the use of high-dielectric (high-κ) materials, pioneered by Intel developments.