Accumulation Region of operation: If a negative voltage is applied at the gate terminal to the p-type doped substrate, then the majority carrier holes will be attracted toward the oxide-semiconductor interface. Also, due to the presence of holes, the interface became more p-type and the valence band will become closer to the Fermi level on the semiconductor side. So upward bending of the band diagram (similar to Fig.3 but in the upward direction) takes place. This is called accumulation and the MOS is in the accumulation region.
Depletion: If a small positive voltage is applied at the gate terminal to the p-type doped substrate, then the majority carrier holes will be repelled back from the oxide-semiconductor interface. As a result, a depletion layer with negative ions is created near the interface. Also, due to the deficiency of holes, the interface became less p-type and the valence band will shift away from the Fermi level on the semiconductor side. So a small downward bending of the band diagram (similar to Fig.3) takes place. This is called depletion.
Inversion: If a large positive voltage is applied at the gate terminal to the p-type doped substrate, then almost all the majority carrier holes will be repelled back from the oxide-semiconductor interface. As a result, a thicker depletion layer with negative ions is created near the interface. Now, the minority carrier electrons are attracted near the oxide interface and create a conducting layer called the Inversion layer. Also, due to the presence of electrons, the interface became more n-type, and more downward bending of the band diagram takes place. This situation is called carrier inversion. When the change in potential near the interface is the same as in the substrate i.e.