Under the assumption of Boltzmann statistics the immobile part of the semiconductor space charge in a MOS structure is calculated. The result of this calculation is compared with the value, which is obtained from a theory of Schottky. This theory yields a square root relation between the immobile space charge and the external device voltage. It is shown that Schottky's theory represents a good approximation to the exact relation. Values are given for the diffusion voltage belonging to the depletion region dependent on substrate doping, external voltage and inversion edge concentration.