The use of electronic computers has increased our knowledge of the electronic structure of solids essentially. The significance of large scale computation can be shown by the development of the semiconductor model within the last twenty years. When — after the discovery of the transistor — an intensive investigation of the properties of Ge and Si began the structure of the conduction and valence bands of these semiconductors was thought to be isotropic and parabolic. This means that the only difference in the properties of the electrons and holes to free charge carriers should be a constant scalar effective mass. The only possible improvement of this simple phenomenological model was a directional dependence of the effective mass. In the frame of this concept a transport theory could be developed in a rough approximation, but a theory of the optical spectra going beyond the explanation of the absorption edge was impossible.
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