Depletion Region Capacitance in MOSFET
In Fig 2.6(c), it's not negative ions that are under the gate, but negative carriers (electrons) that are mobile and free to move in their plane.
Underneath that there is an insulating depletion region (fixed negative charge from the p-type material where holes have been repelled from), and underneath that there is conductive p-type material. The depletion cap is from the top electron layer as one plate; the depletion region as the non-conductive dielectric, and the conductive p-type as the bottom plate.
Because there is a non-movable charge in the depletion region, the electric field in the depletion capacitance is not equal to E = V/d. There is no requirement that the '+' plate have '+' carriers on it.