  Semiconductor surfaces were divided into charge categories, i.e. surface
acceptor, donor and neutral ones that are suitable for simulations of their
properties within a slab model. The potential profiles, close to the charged
surface states, accounting for explicit dependence of the point defects energy,
were obtained. A termination charge slab model was formulated and analyzed
proving that two control parameters of slab simulations exist: the slope and
curvature of electric potential profiles which can be translated into a surface
and volumetric charge density. The procedures of slab model parameter control
are described and presented using examples of the DFT simulations of GaN and
SiC surfaces showing the potential profiles, linear or curved, depending on the
band charge within the slab. It was also demonstrated that the field at the
surface may affect some surface properties in a considerable degree proving
that verification of this dependence is obligatory for a precise simulation of
the properties of semiconductor surfaces.
