Like other surface-potential based model, our surface-potential-plus model starts with charge-sheet approximation, uses the quasi-Fermi-potential to integrate drift and diffusion current and formulates an inversion charge equation that can be analytically solved for given terminal voltage. This eliminates the need for precise computation of the surface potential. Based on the inversion charge solution, a continuous, symmetric and accurate MOS model is developed. Various small dimensional effects including polysilicon depletion, quantum mechanical effects, velocity overshoot, source-side injection limit effect, and quasi ballistic transport of nano-scale MOSFETs are integrated naturally into this model. Comparison with measured data validates the new model. The modeling framework is easily extendable to SOI and double-gate MOSFETs.