The depletion effects of gate poly-Si are investigated in detail taking into consideration the fact that many-body effects due to carrier-carrier and carrier-ion interactions are different at the surface than in a bulk of the gate poly-Si. All calculations are self-consistently performed including an incomplete ionization of activated impurities in an iterative manner. As a result, it is found that the surface part of these interactions affects the equivalent oxide thickness determined by the capacitance - voltage fitting, and that the bulk part affects the determination of flat band potential. It is also found that the surface of the gate poly-Si is incompletely depleted, and the depletion layer is then wider than calculated when assuming the complete depletion (N S /N D ). The width of the incomplete depletion layer is studied in detail for the first time.