In this paper, a charge-based analytical model for intrinsic capacitances in tunnel field-effect transistors (TFETs) is presented. The model is derived for a Si Double-Gate (DG) n-TFET whose flexibility is applicable to single-gate or p-type TFETs as well. The model is verified comparing with the TCAD simulations as well as measurements data. Considering the capacitances of fabricated Si planar p-TFETs on Ultrathin Body, some deviations between TCAD simulations, compact model and measurements are observed. Here the effect of Schottky barriers at NiSi2 contacts are analyzed and a theory for the reason associated deviations and the unexpected behavior of intrinsic capacitances is evolved. Furthermore, a technique to include this effect in the aforementioned model is also presented.