Polysilicon depletion effect

The poly layer is doped with N-type or P-type impurity to make it behave like a perfect conductor and reduce the delay.

As a result of the separation of charges a depletion region is formed on the polysilicon-oxide interface, which has a direct effect on the channel formation in MOSFETs.

Polysilicons were used as their work function matched with the Si substrate (which results in the low threshold voltage of MOSFET).

[9] So the effect with doped poly is an undesired reduction of threshold voltage that wasn't taken into account during circuit simulation.

In order to avoid this kind of variation in vth of the MOSFET, at present metal gate is preferred over Polysilicon.