# Calculating Capacitance

1. ## cox capacitance

Hi,
Well this time I have w and l values of a transistor whose source and drain are shorted to form a Capacitor. I need to calculate the capacitance.
Can any one give me expression for this?

Thanks,
Gold_kiss

2. ## eox + capacitance

Assuming that you have the transistor in strong inversion, i.e., VGS>Vt+0.2 (approximated value), the capacitance is

Cap=W.L.Cox

where Cox is the gate capacitance per unit of area:

Cox=eox/tox

eox-Sio2 dielectric cte
tox-oxide thickness

Regards

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3. ## how to calculate gate capacitance

Do you want to calculate the capacitance or measure the capacitance if the case of measurement make a oscillator out of it and find it out if you know the specifications the then above mentioned formula can be used

4. ## how to calculate drain capacitance

hi,maxwellequ
You just compute the gate capacitor C1, however, you omit the capacitor C2 between the channel and bulk.
Detailed equation is on the Razavi's analog book.

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5. ## eox capacitance

Dear flyinspace,

If the transistor is in strong inversion, then the inversion layer is formed. In that case the two plates of the MOS capacitor are that inversion layer (which is accessed by drain/source terminals) and the gate. There is, of course, some capacitance between the channel and the bulk. However in the usual case of MOS capacitors, where the drain/source terminals are connected to the bulk, this can be neglected.

If the MOS is NOT in strong inversion, then things are a little bit more complicated and you are right. Razavi book presents a "light" discussion. For details refer to Tsividis book (Operation and Modeling of the MOS transistor).

Regards

6. ## capacitance cox

You also can google how to calcualte the capacitance you want to have. Lots of online notes are talking about that.

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7. ## calculating gate capacitance

Well,
here is a idea i can give u, w.k.t i = c* dv/dt , apply a unit ramp i/p
to gate of MOS, and measure current, it will be equal to the cap in magnitude.

rgds,

8. ## calculate capacitance in analog

hi,airace
Can you tell me what is the meaning of a unit ramp i/p? what is p?

9. ## calculate gmt and cte

Hi,

You can use not only strong inverse MOS capacitor.
May be better to use capacitor w/o inverse layer, for example, NMOS in NWell (N+ drain and source in NWell). It's so called Accumulation MOS (AMOS) capacitor.
It's easy to simulate this capacitor in SPICE - use PMOS model, node connection - Gate and Bulk.

More detail you can see in
T.C.Lin, J.C. Wu, Implementing Compensation Capacitor in Logic CMOS Process, p.1642-1650, IEICE Trans. Electron., v. E85-C, No.8, August 2002

Regards,

10. ## Re: eox + capacitance

Originally Posted by maxwellequ
Assuming that you have the transistor in strong inversion, i.e., VGS>Vt+0.2 (approximated value), the capacitance is

Cap=W.L.Cox

where Cox is the gate capacitance per unit of area:

Cox=eox/tox

eox-Sio2 dielectric cte
tox-oxide thickness

Regards
For the tox, is it the electrical tox listed in the tsmc? or the fox?

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