Geoffrey_Coram wrote on Aug 30th, 2006, 6:42am:For a moscap structure, I would indeed expect Cgb=Cbg; the gate charge must be equal and opposite the channel charge for charge neutrality.
Have you partitioned the channel charge into d, s, and b parts? Do you have Qd and Qs non-zero? In a MOS, if Qd=Qs=0, then Qb = -Qg, and it seems you're back in the moscap situation ...
Yes, my model has non-zero qd and qs at strong inversion. However, my confusion is not with the model, but rather the simulation results shown in MEDICI simulation. Please find the attached. The Cbg is following Cgb, at Vds=0, which i suppose to be different from what i thought. Though Vds=0, i suppose it will still be different as there are still inversion charge at Vds=0 and some parasitic effects, such as the overlap charge should be added to the gate-charge. This device is very short, about 90nm(channel length), overlap length=20nm. It seems to me that the parasitic charges due to overlap is not included in the gate? or is it included somewhere else???