21.3 Metal-Insulator-Semiconductor Diodes
547
Fig. 21.31 Band diagram of ideal MIS diodes with a, b, c n-type and d, e, f p-type semiconductors for V = 0 in
stationary equilibrium for the cases a, d accumulation, b, e depletion and c, f inversion
In Fig. 21.35e the depletion case is shown. Now a moderate reverse voltage, i.e. a positive bias to
the metal, is applied. A depletion of majority charge carriers occurs in the semiconductor close to
the insulator. The quasi-Fermi level in the semiconductor remains beneath the intrinsic level (E i ≈
E C + E g /2), i.e. the semiconductor remains p-type everywhere. If the voltage is increased further to
large values, the quasi-Fermi level intersects the intrinsic level and lies above E i close to the insulator
(Fig. 21.35f). In this region, the electron concentration becomes larger than the hole concentration and
the inversion regime is reached. The inversion is called ‘weak’ if the Fermi level is still close to E i .
The inversion is called ‘strong’ when the Fermi level lies close to the conduction-band edge with an
accordingly large inversion charge.