A physics-based model for the electric potential of vertical TFET is presented in this article. The electric potential formula is derived for the first time from the multi-branch general solutions of Poisson's equation for TFET. The effect of electron inversion charge in the channel is taken into account. A novel approach incorporating the effect of hole mobile charge in the source depletion region is proposed. The model's accuracy is significantly improved compared with the previous source's fully depleted approximation. The model is proven to be accurate in all operating regions. The model's results are verified with TCAD simulation for different structural and material parameters.