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Question about NMOS capacitor

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Nmos used for capacitor,
M1: W=20u,L=5u, m=80
M2: W=20u,L=20u, m=20

The area of M1 and M2 are equal,
area M1=20*5*80=8000
area M2=20*20*20=8000

Questions are :
(1)Which one would be better for the capicitor?
(2)Capcitor M1 equals Capcitor M2?or not?why?

Thanks.
 

Re: About mos capacitor

(1)Which one would be better for the capicitor?
Click to expand...

If there is only a capacitor, I think both make no difference. If you want to match 2 cap with the same dimension, square structure will be better.

(2)Capcitor M1 equals Capcitor M2?or not?why?
Click to expand...
Considering sidewall cap M2 has bigger cap.
 

    G

    gxxg

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Re: About mos capacitor

Then how to know the size of this mos capacitor?That is ,how much it will be for a 20/20,M=20 Nmos ?10p?20p?
How to calculate or simulate?

Thanks.
 

Re: About mos capacitor

Nmos used for capacitor,
M1: W=20u,L=5u, m=80
M2: W=20u,L=20u, m=20

The area of M1 and M2 are equal,
area M1=20*5*80=8000
area M2=20*20*20=8000

Questions are :
(1)Which one would be better for the capicitor?
M1 is better, because the Length is smaller, so the resistance of MOS capacitor is smaller.
(2)Capcitor M1 equals Capcitor M2?or not?why?
The effective capacitance : M1>M2, because L of M1 is smaller
 

Re: About mos capacitor

I think they are same in using as capacitor by gate to drain and source
 

Re: About mos capacitor

jamesko said:
Nmos used for capacitor,
M1: W=20u,L=5u, m=80
M2: W=20u,L=20u, m=20

The area of M1 and M2 are equal,
area M1=20*5*80=8000
area M2=20*20*20=8000

Questions are :
(1)Which one would be better for the capicitor?
M1 is better, because the Length is smaller, so the resistance of MOS capacitor is smaller.
(2)Capcitor M1 equals Capcitor M2?or not?why?
The effective capacitance : M1>M2, because L of M1 is smaller
Click to expand...

If we put them in parallel using the 20u side then we will have 4 times more fringing capacitance for M1 compared with M2
 

About mos capacitor

For MOS capacitance,does W or L make any difference???
I think the equ value has sth to do with their perimeter
 

Re: About mos capacitor

M1 is better as its ESR is smaller due to shorter length
 

Re: About mos capacitor

If the Q of the Capacitor is important, then you should use big W/L values to increase the Q of the Capacitor. Therefore if you need High Q Cap, you may use the first one.

Rule of thumb: At least w/l~10
 

Re: About mos capacitor

you would need to define what qualities you deem important before you can decide which is 'better'. Incidentally, what is the lowercase m stand for? I design CMOS camera chips for a living and I use NMOS devices as capacitors everywhere. Our foundry state the capacitance to be a function of W and L.
 

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