MOSFET capacitances - which ones matter most?

[diarmuid]

Hello,

I am looking for infomation about the more dominant parasitic caps in a MOSFET.

Would I be correct in saying the most dominant parasitic caps are the following:

Cgs
Cgd
Csb
Cdb

If so,

1.) Is Cgs >> Cgd? I got the below equations from a previous post. Are they correct? If so
I guess that would explain it!

CGD(total)=Cox∙W∙LD
CGS(total)=(2/3) Cox∙W∙ L +Cox∙W∙LD

2.) Is Csb >> Cdb or vice versa? I have no idea about this one!

3.) Are there any other parasitic caps I should also be considering?

Thanks,

Diarmuid

 

[kripacharya]

Cgs is usually the one most talked about, and needing to be designed around.

But it depends on what/ where/ how you are using this mosfet.

 

[Electro nS]

Cgs is not parastic , it is alsmost real capacitance and is the one that matters the most .

 

[dick_freebird]

This depends somewhat on the close-in circuit topology.

A low current common-source amplifier will see Cgd as
its bandwidth limiter, Miller gain is the problem especially
when the gate is driven by some weak current source.
A common-drain buffer stage would see Cgd as a less
significant penalty.