polySi

PolySi, short for polysilicon, is a polycrystalline form of silicon composed of small crystalline grains. It is dopable to n-type or p-type by introducing phosphorus or boron, making it conductive rather than intrinsic silicon. PolySi is produced by chemical vapor deposition, typically using silane (SiH4), at high temperatures. The resulting film is subsequently doped and annealed to activate dopants and improve crystal quality.

In semiconductor manufacturing, PolySi has been widely used as a gate electrode in MOS devices and as a diffusion barrier or resistor. PolySi gates were standard from the 1980s through the early 2000s, offering compatibility with silicon technology and the ability to tune threshold voltages through doping. However, PolySi gates suffer from depletion effects and resistivity limitations. The introduction of metal gates and high-k dielectrics in advanced nodes reduced reliance on PolySi as a gate material. In newer processes, PolySi may still be used for non-gate roles or in legacy devices; it can be silicided on the surface to form silicides and reduce resistance, a common practice in self-aligned silicide processes.

Properties of PolySi include chemical stability, compatibility with silicon substrates, and the ability to produce controlled dopant profiles. Its resistivity depends on dopant concentration, and high-temperature processing can affect grain structure and surface roughness. Limitations include dopant diffusion, grain boundary scattering, and depletion effects that can reduce drive current in a transistor.