This question is somewhat relating to electronics but I think it fits better with physics. The leccture here (slide 13) shows that the gate to body capacitance (Cgb) in linear region is 0. However, there is no explanation at all. I tried to find the explanation from the reference book and got this: In linear-mode operation, the inverted channel extends across the MOSFET, between the source and the drain (Fig. 3.31(b)). This conducting inversion layer on the surface effectively shields the substrate from the gate electric field; this Cgb = 0. With the shielding, does this mean the electric field in substrate is 0? Is there a way to prove that Cgb = 0 mathematically? Thank you.