Three different operating regions can be observed  for the MOS system.They are:

(1) Accumulation

(2) Depletion

(3) Inversion

(1) Accumulation

a. If a negative voltage $\large V_G$ is applied  to the gate electrode,the holes in the p-type substrate are attracted to the semiconductor oxide interface.

b.The majority charge carrier concentration near the surface becomes larger than the equilibrium hole concentration in the substrate,hence this condition is called carrier 'ACCUMULATION' on the surface.

c.The negative surface potential also causes the energy bands to bend upward near the surface.While the hole density near the surface increases as a result of the applied negative gate bias,the electron concentration decreases as the negatively charged electrons are pushed deep into the substrate.

(2) DEPLETION