A layer of silicon oxide (SiO 2) typically grows on a silicon substrate and a metal or polysilicon layer is deposited over it. If you observe, this acts as a planar capacitor, in which the metal (gate) and the semiconductor (body) are separated by a dielectric (silicon oxide).
Therefore, when a potential is applied to the gate, opposite charge carriers are accumulated at the interface of the oxide body. If the potential at the gate is further increased, charge carriers are drawn from the source into this depleted layer, which causes an inversion layer on the body.
It is therefore necessary to have an insulator layer at the gate connection in order to produce this inversion layer. This insulation layer increases the input impedance, making it more sensitive to voltage fluctuations.
The MOS capacitance is basically known as the heart of the MOSFET. Due to the presence of this insulating layer between the gate metal plate and the semiconductor substrate, a parallel plate capacitor is formed (the metal and the semiconductor acting as two plates of the capacitor).
This capacitive action thus formed enables the gate voltage to control the width of the channel, which in turn controls the amount of current flowing through the device. Because of this working principle, Mosfet becomes a voltage-controlled device, with the control connection being the gate.