Polysilikongater

Polysilikongater, or polysilicon gates, are gate electrodes used in metal-oxide-semiconductor (MOS) transistors. They consist of a polycrystalline silicon layer that forms the gate which sits above the gate dielectric, typically silicon dioxide or a high-k dielectric in advanced devices. Polysilicon was the standard gate material in most CMOS processes for several decades because it is compatible with silicon processing, its electrical properties can be tuned by doping, and it allows the formation of low-resistance contacts through silicide formation.

Fabrication and materials: Polysilicon gates are deposited as a doped polycrystalline silicon film by chemical vapor deposition, then doped with n-type or p-type dopants (such as phosphorus or boron) to control conductivity and the work function. The layer is patterned by photolithography and etching to form individual gates, and a subsequent anneal activates dopants, influences grain structure, and helps crystallization. The work function of the gate can be adjusted via dopant type and concentration, which in turn influences the transistor’s threshold voltage.

Electrical properties and limitations: A notable issue with poly-Si gates is poly depletion, where the gate loses some capacitance under bias, reducing gate control and drive current. Gate resistance can also be higher than with metal gates. To mitigate resistance, silicide formation (e.g., TiSi2, CoSi2, NiSi) is used to lower contact resistance. As device dimensions shrank and the need for better electrostatics grew, metal gates and high-k dielectrics were introduced in many processes, leading to a gate-last or replacement metal gate (RMG) approach. In such schemes, poly-Si may be used as a sacrificial layer during fabrication and removed in favor of metal gates.

Today, polysilicon gates remain a foundational concept in MOS technology but have been largely supplanted by metal gates with high-k dielectrics in many modern deep-submicron processes.