AN2386 datasheet(5/30 Pages) STMICROELECTRONICS

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AN2386

MOS structure

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MOS structure

As it is well known, a MOS structure is composed by three layers: the first one is metal or

heavily doped polycrystalline silicon, the second one is an insulator of SiO2 and the third

one is the semiconductor (see Figure 1.).

Figure 1.

Cross section view of a MOS capacitor

Considering an ideal MOS system with a p-doped semiconductor, the energy band diagram

can be illustrated as in Figure 2.

Figure 2.

Energy band diagram of an ideal MOS capacitor under thermal

equilibrium.

Φm is the work function (energy that needs to extract an electron from the metal); q ΦB is

the energy difference between the oxide conduction band and the metal Fermi energy level

(metal-to-oxide barrier energy); q

Φsc is the work function of the semiconductor; q χ is the

energy difference between the vacuum level and the conduction band edge.

When a negative voltage (Vg) is applied on the gate terminal respect to the semiconductor,

the Fermi level of the metal raises of qVg compared to the semiconductor side. In moving

from the semiconductor to the metal, the vacuum level must bend up gradually to

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