AN2386 datasheet(7/30 Pages) STMICROELECTRONICS

7 / 30 page

AN2386

MOS structure

7/30

The charge Qs in the semiconductor side near the oxide interface is equal to Qg and it can

be written as:

Equation 1

Na is the acceptor concentration and W is the width of the surface depletion layer. Increasing

the gate voltage the bands continue to bend downward until Ei (the intermediate level

energy between the conduction and the valence bands) equals EF (the Fermi energy level)

at the surface. In this condition, at the interface near the oxide, the semiconductor becomes

intrinsic. Increasing the gate voltage again, Ei crosses EF and, thus, the minority carriers, in

this case the electrons, are attracted to the oxide-semiconductor interface. In this new

condition, the surface layer contains more electrons than holes and it becomes n-type

(inversion layer) (see Figure 5.).

Figure 5.

Energy band diagram and charge distribution in an ideal MOS capacitor

in inversion condition

Now, Qs can be written as:

Equation 2

Qn is the inversion layer charge.

The relationships between the band bending, the electron and hole concentrations in the

interface oxide-semiconductor may be obtained assuming that the semiconductor is non

degenerate and that the doping is uniform. The electron and hole concentrations on the bulk

semiconductor can be written respectively as:

Equation 3

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