1.The MOS structure consists of three layers

(a)The metal gate electrode

(b)The insulating oxide layer (SiO2)

(c)The p-type bulk substrate (Si),called the Substrate.

2.As such,the MOS structure forms a capacitor,with the gate and the substrate acting as the two terminals and the oxide layer acting as the dielectric.

The figure below shows the energy band diagrams of metal,oxide and semiconductor layers in a MOS system as  three separate components

3.Now,consider the three components of the ideal MOS system are brought together into physical contact.The Fermi levels of all three materials must line up,as they form the MOS capacitor.

4.Because of the work function difference between the metal and the semiconductor,a voltage drop occurs across the insulating oxide layer.

5.The rest of the voltage drop occurs at the silicon surface next to the silicon-oxide interface,forcing then energy bands of silicon to bend in this region.

6.The equilibrium Femi levels of the substrate and the metal gate are at the same potential.Also,the Fermi potential at the surface($\phi_S$) is smaller in magnitude than the bulk Fermi potential $\phi_F$

The diagram below shows the combined  energy band  diagram of the MOS system