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Are Cgd and cgdo the same thing?

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Log!c

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[note: I wrote the post topic wrong and now it isn't changing. I meant Cgd and cgdo!] Fixed - Keith

Hi everyone,

1) I'm still fairly new at simulating things in cadence. In my book the gate to drain capacitance is listed as Cgd. Is this the same as cgdo (gate drain overlap capacitance) in a spice model?

2) Also, as a side question, I was ran a DC-op analysis, and then went into some browser where you could look at a lot of different values for different transistors. You could right click on a property and view it in a "table". Some of the capacitance values (or so I thought) were negative. What's the deal with this 8-O
 
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Cgd0 is the overlap capacitance between the gate and drain which is always present and varies little AFAIK. It is different from cgd. You need to follow the cgd parameter. It is the actual cgd which is given in the text books. cgd is calculated parameter from the operating conditions of transistor and some other channel charge distribution modles selected in the model file. You might be using the BSIM3 model for your simulation which is widely used in many PDKs.
 
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