The paper describes a novel approach in approximate solving the implicit relation between the surface potential and terminal voltage in n-type MOS transistor. Using the homotopy perturbation (HP) method, an approximative solution, which at the same time has a relative simple mathematical form, and a high degree of accuracy was obtained. It is shown that HP approximations, in comparison to a previously used approximate methods, have a many advantages: they can be calculated recursively to arbitrary numerical precision, and the whole procedure can be implemented by using the appropriate software.