[help] ac-coupliing capacitor can be used mos capacitor?

Compare to mim capacitor , mos capacitor can save much of area, but in circuit design i seldom see people used it , most use mim capacitor ,what is the reason? compare to mim capacitor , what is the disadvantages of the mos capacitor ?

Re: [help] ac-coupliing capacitor can be used mos capacitor

Hi,

Actually what is mim capacitor, can you plz expand /explain it..With the Mos capacitors if we see, though area wise we get the adv..I think in stability ( like leaking and other issues) arer not good with it..

Regards,
GD

linearty

MOS capacitance has a large voltage coefficient

Re: [help] ac-coupliing capacitor can be used mos capacitor

thank you all!
mim is metal-insulator-metal

Re: [help] ac-coupliing capacitor can be used mos capacitor

skyeaglemm said:

Compare to mim capacitor , mos capacitor can save much of area, but in circuit design i seldom see people used it , most use mim capacitor ,what is the reason? compare to mim capacitor , what is the disadvantages of the mos capacitor ?

Click to expand...

mos capacitor need voltage difference larger than threshold.

the main diffrenece is ideality of the cap. as ideal cap should have constant value with respect to the applied voltage, which is not true in MOS cap.

As it's seems to me for prevention of CDM breakdown the mos tr. like ac-coupliing capacitor aren't used. For provide logic "1" of "0" in mos like technology the gates of mos tr. don't connect direct to supply rail, and its connect thought RC (for prevent CDM breakdown). Can anybody explain i'm right?

Re: [help] ac-coupliing capacitor can be used mos capacitor

Hi,

I also have this doubt

safwatonline said:

linearty

Click to expand...

safwat, what do you mean by this? do you mean voltage coefficient only?

pfd001 said:

MOS capacitance has a large voltage coefficient

Click to expand...

As I know, voltage coefficient of moscap in strong inversion is low enough for appliations such as opamp compensation cap. But I rarely come across people using the mos cap for opamp compensation even in the cases where strong inversion for mos is gauranteed. Instead people use high area alternatives like mim and poly caps. I think, even the variation of moscap is low across the corners when compared to mim and poly.

But why people don't prefer mos cap?

Re: [help] ac-coupliing capacitor can be used mos capacitor

the premise for mos capacitor is that it has to operate in strong inversion.
that's to say, the voltage difference between the two nodes of the mos capacitor is rather large, that's the main reason for limiting its application.
in short, the introduce of mos capacitor will influence the dc operating point of the circuit.

Re: [help] ac-coupliing capacitor can be used mos capacitor

hello,
i meant this

safwatonline said:

the main diffrenece is ideality of the cap. as ideal cap should have constant value with respect to the applied voltage, which is not true in MOS cap.

Click to expand...

I have joined today - and MOSCAP was a random first hit.

Great discussion, regarding the C-V non-linearity of MOSCAP especially near Vth..

my two cents..

1.
MOSCAP - one of the electrode is POLY the other one is the channel (connected to source-drain-shorted-together) - and in MOSCAP usually the length of the channel is much larger than switching MOS (usually MOSCAP channels are squares - same width/length ). The displacement current through the capacitor - reaches metal1 interconnect at the shorted-source-drain contact by drift and diffusion of carriers through the channel - not as good conductance as a metal plate.
Hence this is like:
<Gate-term>---rr--C--rrrrrrrrrrrr---<S_D-term>

by r I meant resistance.

[ Aditional channel implantation makes MOSCAP behave little better ]

MIM structure on the otherhand offers more purer C - less terminal res.


2.
At the lower technology node >90nm MOS gate-oxide is leaky, hence MIM got preference.