Refering to fig. 4.1, we can see there is an oxide layer below the Gate terminal. Since oxide is a very good insulator, it contributes to an oxide capacitance in the circuit.
Normally, the capacitance value of a capacitor doesn't change with values of voltage applied across its terminals. However, this is not the case with MOS capacitor. We find that the capacitance of MOS capacitor changes its value with the variation in Gate voltage. This is because application of gate voltage results in band bending in silicon substrate and hence variation in charge concentration at Si-SiO2 interface.
