Poly-Poly capacitor vs MOS capacitor

Why is the capacitor/unit area value of the Poly-Poly capacitor typically less than that of MOS capacitor?

Because thickness of gate oxide is less, than poly-poly oxide.

gate capacitance is the largest capacitance due to the small oxide thickness

what's the advantages of p-p cap versus mos cap?
linearity?

yes , more linearity ( Vs. voltage across cap.)

yes,linearity across voltage

the p-p capacitor more currate than MOS capacitor

Think of the process fabrication whenever possible in understanding layout.
POLY is always grade A photomask, meaning the one of the highest ranking in photomask fabrication, same as some metals. Diffusion is instead not, also, it is formed from doping concentration by ion implantation, which is typically not very good in control compared with poly, so the profile is not as good as POLY. PP cap is formed from two POLY layers, as a result, the oxide built in between is more uniform that the oxide formed from gate to diffusion. However, as gate oxide tends to reduce with decreasing technology, so MOS cap tends to be much larger than PP cap, some multiples to PP cap is not uncommon.
However, MOS cap has to be biased with a gate voltage such that the MOS cap is being constant, which is also a major factor for its value being varied from PVT.

Remember there is voltage and temperature effect from PP cap as well

MOS cap has a big voltage coefficient

hung_wai_ming(at)hotmail.com said:

...PP cap is formed from two POLY layers, as a result, the oxide built in between is more uniform that the oxide formed from gate to diffusion...

Click to expand...

As I know ,isn't the gate oxide most carefully controlled in depth??It's critical to the mos transistor's performence.

Yes,MOS cap is voltage dependent.

Hi leohart,

oxide built on top of diffusion can be as good as built on poly. Also, most of the time, transistors are not drawn in square, but is PP cap.

Yes, I check the mos structure,and the oxide right under gate is uniform,but at the edge it tends to be a littele thicker,maybe that's your point...

Since Poly-Poly oxide thickness is thicker than MOS cap oxide. Cap=EA/d dxide thickness