Channel to bulk capacitance in MOSFET during strong inversion

[Analog VLSI enthusiast]

The total capacitance of the MOSFET does not take into account the capacitance between channel and bulk during strong inversion. WHy?

 

[sutapanaki]

Well, simplistically said, because if bulk is connected to ground and source is too, then the channel, which is an extension of the source has a capacitance to bulk that's shorted. On the drain side, in saturation you have the pinch-off, which is basically a depletion region i.e. no channel, so no capacitance. Realistically, the tip of the channel has different potential compared to the source so some capacitance to bulk too, but compared to all other caps in the device this should be pretty insignificant.