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I am using sentaurus tcad simulator for simulating a gate all around nano wire and nano tube mosfet. I have to extract the total gate capacitance Cgg and its components like Cgs, Cgd etc wrt gate voltage. I have done ac simulation at 1 MHz. The total capacitance Cgg is a positive value, but while looking at the different components like Cgs, Cgd their values are coming to be negative. Even for reverse polarity capacitance like Csg, Cdg their values are different compared to Cgs and Cgd and negative also. Is the negative values possible and which components should I see for gate-to-source capacitance and gate-to-drain capacitance in the simulator?

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  • \$\begingroup\$ A negative capacitance is usually regarded as an inductor. \$\endgroup\$
    – Andy aka
    Jun 4 '18 at 9:31
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Disclaimer: I have no personal experience with the CAD software you're using.

Searching the internet however, does give some clues.

Reference 1

Reference 2

I believe that the most plausible reason for your results, is that the definition TCAD is using is the following:

$$C_{xy,TCAD} = \frac{dQ_x}{dV_y}$$

Where \$dQ_x\$ is the total amount of charges that are added on the capacitances at the \$x\$-terminal when terminal \$y\$ changes voltage (\$dV_y\$).

So this means that

$$C_{gs,TCAD} = \frac{dQ_g}{dV_s}$$

Which is not the same as the typical definition of capacitances

$$C_{gs,typ} = \frac{dQ_{Cgs}}{dV_{gs}}$$

Where \$Q_{Cgs}\$ is the charges stored on the capacitance between gate and source, while \$V_{gs}\$ is the voltage over that capacitance.

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