The Ideal MOS Capacitor
The surface effects that arise in an apparently simple MOS structure are actually quite complicated. Although many of these effects are beyond the scope of this discussion, we will be able to identify those which control typical MOS transistor operation.
Band diagram for the ideal MOS structure at different operating condition:
- Some important definitions are made in the energy band diagram of Fig.(a). The work function characteristic of the metal can be defined in terms of the energy required to move an electron from the Fermi level to outside the metal.
- In MOS work it is more convenient to use a modified work function qΦm for the metal-oxide interface.
- The energy qΦm is measured from the metal Fermi level to the conduction band of the oxide.
- Similarly, qΦs is the modified work function at the semiconductor-oxide interface.I
- In this idealized case we assume that Φm = Φs so there is no difference in the two work functions
- The MOS structure of Fig.(a) is essentially a capacitor in which one plate is a semiconductor.
- If we apply a negative voltage between the metal and the semiconductor (Fig.b), we effectively deposit a negative charge on the metal.
In response, we expect an equal net positive charge to accumulate at the surface of the semiconductor. In the case of a p-type substrate this occurs
by hole accumulation at the semiconductor-oxide interface
- Since the applied negative voltage depresses the electrostatic potential of the metal relative to the semiconductor, the electron energies are raised in therelative to the semiconductor.
- As a result, the Fermi level for the metal EFm lies above its equilibrium position by qV, where Vis the applied voltage.