Metal gate

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.

For decades (i.e., in the late 1970s), the industry had moved away from metal (most typically aluminum, evaporated in a vacuum chamber onto the wafer surface), as the gate material in the MOS stack due to fabrication complications. A material called polysilicon (polycrystalline silicon, highly doped with donors or acceptors to reduce its electrical resistance) was used to replace aluminum because:

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.

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

The candidates for the metal gate electrode are probably, for NMOS, Ta, TaN, Nb (single metal gate) and for PMOS WN/RuO₂ (the PMOS metal gate is normally composed by 2 layers of metal). Why two metal gate? Simply because thanks to this solution, we can improve the strain capacity on the channel (by the metal gate). Moreover, this enables less current perturbations (vibrations) in the gate (thanks to the disposition of electrons inside the metal).

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