An improved numerical computation model is presented to calculate the metal-ferroelectric-semiconductor field effect transistor (MFSFET) characteristics with a sufficiently large gate area which can be applied for large drain voltages. In the presented model, the polarization of the ferroelectric gate insulator is inhomogeneous and treated as a variable along the channel. We have calculated electrical properties of SrBi2Ta2O9/Si MFSFETs and demonstrate that the conventional model, in which the polarization of the ferroelectric gate insulator (Pd) is treated as a constant, overestimate the drain current when the drain voltage is large. In addition, effects of the ratio of remanent polarization to spontaneous polarization (Pr/Ps ratio) of the ferroelectric film on the transistor characteristics are discussed.