Charge Distribution in Surface States

At a semiconductor surface the crystal lattice is interrupted. In consequence surface states within the bandgap may exist at the surface as already discussed in Chapter 2. Charges can be stored in surface states. The occupation by electrons depends on the position of the Fermi level with respect to such a surface state. The charge is given by ess=e/M (5.35) 

Since the distance between the Fermi level E and the energy bands varies with (see Section 5.3) f is also changed. If Fp = Ft, the surface state is half-occupied (f = 0.5), as shown in Fig. 5.8. Since the charge depends on the potential across the space charge layer, a differential surface state capacity can be defined by 

As can be recognized very easily, d f/(dA ) and therefore passes a maxi- 

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