By considering the finite distance of the semiconductor space‐charge center away from the Si‐SiO2interface, a modified expression of the flat‐band voltage (VFB) due to the surface charge density (Qss) is derived. For the accumulation and strong inversion cases, the conventional expressionQss/Co, is valid, however, under the depletion condition the modified expression will be used. TheVFBis found to be nonlinearly related toQss, and the expression may be deduced to the conventional form only in the limit that theQssis low, the semiconductor is heavily doped, and the oxide thickness is large.