The attached figure shows the energy diagram of the MOS system for n-Ge in accumulation. Positive bias on the gate attracts electrons to the Ge surface. The surface charge will consist of an accumulation layer of electrons with a density at the surface greater than in the bulk. How to calculate the accumulation layer width?
Look at the standard theory for the MOS system in the book by Sze, or Taur.
You solve Poisson's equation to get the electric field; you can then set up an integration to get the thickness.
Dear Min,
there is also, in addition to Stephen's suggestion, a book directly on the subject :
MOS Physics and Technology by Nicollian and Brews.
All of these though are more or less, analytical approximations to the problem at hand. If you need a quick answer for the width of the accumulation layer, then it is Debye screening length Ld, as shown in the attached figures. If I remember correctly, it is in fact forward bias independent for sufficiently large biases. This though does not mean that the space charge density is bias independent !!
If you would need precise answer, I would have to do the numerical simulation again.
I hope this will help
With best regards
Petr
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