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29th March 2004, 15:48 #1
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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

29th March 2004, 16:04 #2
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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
eoxSio2 dielectric cte
toxoxide thickness
Regards

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30th March 2004, 03:53 #3
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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

30th March 2004, 03:56 #4
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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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30th March 2004, 11:55 #5
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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

30th March 2004, 17:54 #6
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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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31st March 2004, 05:52 #7
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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,

31st March 2004, 14:57 #8
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calculate capacitance in analog
hi,airace
Can you tell me what is the meaning of a unit ramp i/p? what is p?

1st April 2004, 10:49 #9
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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.16421650, IEICE Trans. Electron., v. E85C, No.8, August 2002
Regards,
Uladz55

23rd December 2010, 21:13 #10
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