In the presence of a semiconductor surface inversion layers thereon along the surface can be current, i.e. they can be channels of surface electrical conductivity. In the evaluation of the channel conductivity, surface conductivity must be considered that the concentration of charge carriers is variable according to the depth of the channel and the mobility of charge carriers in the channel can differ significantly from the mobility of the same carriers in the bulk of semiconductor due to the additional scattering of carriers at the crystal surface.