can anyone explain simply about poly Si depletion

can anyone explain simply about poly Si depletion:?:

Polysilicon depletion effect is the phenomenon in which unwanted variation of threshold voltage of the MOSFET devices using polysilicon as gate material is observed, leading to unpredicted behaviour ( lead to the increase of the potential drop and degradation in device performance). Polycrystalline silicon, also called polysilicon, is a material consisting of small silicon crystals. It differs from single-crystal silicon, used for electronics and solar cells, and from amorphous silicon, used for thin film devices and solar cells.
Polysilicon electrical depletion in a polysilicon gate electrode is reduced by depositing the polysilicon under controlled conditions so as to vary the crystal grain size through the thickness of the polysilicon. Besides that, polysilicon depletion effect can be improved by reducing gate doping concentration (Np).

https://en.wikipedia.org/wiki/Polysilicon_depletion_effect

Just as the gate "pushes" on the body - accumulating,
depleting or inverting it - so does the body "push" on the
gate. If there is a depletion region in the poly, that adds
to the effective Tox, in a sense.

Gate poly implanted from above, tends to have less
dopant at the top gate interface (you could position
the peak at the interface, but then would have a very
large straggle into the oxide, doing implant damage
and probably ruining reliability).