I just thought of sharing some basic information of gate in mosfet, Why do we go for polysilicon gate rather than metal which is very good conductor. Two reasons i know i have written here.
First thing is the threshold voltage(Vt) variation is quite difficult in case of metal gate mosfets, as threshold voltage of a mosfet depends on the work function difference of gate and substrate. The work function of metals cannot be easily varied so its challenging to to tune the work function of metal to reduce the threshold voltage. Also using two different metals to reduce Vt for pmos and nmos makes the process complex. The Vt can be modified by the work function difference between the gate material and channel material. Because polysilicon is a semiconductor, its work function can be modulated by adjusting the type and level of doping. And polysilicon has the same as the underlying silicon channel, it is quite straightforward to tune the work function to achieve low threshold voltages for both NMOS and PMOS devices.
And secondly in the process, it is preferable to deposit the gate material prior to certain high-temperature steps in order to make better-performing transistors. Such high temperature steps would melt some metals, limiting the types of metal that can be used in a metal-gate-based process.
The use of a poly-silicon gate has the disadvantage that the sheet resistance of the gate is much larger than that of a metal gate. This leads to high RC time-constants of long poly-silicon lines. The resistivity can be lowered by increasing the level of doping. To improve conductivity a high-temperature metal such as
