Geoffrey_Coram
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MOS models have four "buckets" of charge; depending on the four terminal voltages (D, G, S, B) and hence the operating region, you get different amounts of charge in each bucket.
Cxy tells you how much the charge in bucket "x" (Qx) changes when you wiggle the voltage on terminal "y" (Vy).
You really can't expect that the change in Qd when wiggling Vg would be the same as the change in Qg when wiggling Vd.
However, charge is conserved, Qd+Qg+Qs+Qb=0, and there are only 3 independent branch voltages, so you get identities like Cgg=Cgs+Cgd+Cgb. Cdd=Cdg+Cdb+Cds and you can also write Cdd=Cgd+Csd+Cbd. (Note that Cxy = -dQx/dVy when x!=y.)
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