Difference between Poly cap and transistor used as a cap?

[Electronicmember]

Hello,

usually poly-to-Poly capacitors are used.

But sometimes, the capacitance between the gate the bulk of a transistor(bulk, source, gate are shorted) is also used a cap

I am thinking what are the differences between these two kinds of capacitors? Besides that the transistor needs less space.

Thank you

 

[ammarzouk]

Re: Difference between Poly cap and transistor used as a cap

The difference is in the capacitance per unit area for both kinds of caps. Also, depending on the process and technology, the variation in the capacitance and parasitic resistance associated could be quite significant depending on which cap you use. Finally, the size of the MOS cap is usually smaller than that of the poly-poly cap.

 

[jinjin0412]

Re: Difference between Poly cap and transistor used as a cap

Hello,

usually poly-to-Poly capacitors are used.

But sometimes, the capacitance between the gate the bulk of a transistor(bulk, source, gate are shorted) is also used a cap

I am thinking what are the differences between these two kinds of capacitors? Besides that the transistor needs less space.

Thank you

Poly-poly cap is almost constant with zero voltage coefficient, MOS cap is nonlinear with voltage coefficient and saving a layer mask of poly2(this is usually important), good designers can always use MOS caps