This paper outlines the charge-based core and the architecture of the BSIM5 MOSFET model for sub-100nm CMOS circuit simulation. The BSIM5 model is a continuous, completely symmetric and accurate non charge-sheet based MOS transistor model derived from the basic device physics including various physics effects. Comparison of the inversion charge between the BSIM5 prediction and self-consistent numerical solution shows good agreement in this paper. The demonstration of fully symmetry characteristics of BSIM5 in Gummel Symmetry Test, such as channel current and its high-order derivative, and charge and trans-capacitances in SPICE simulation, also implies BSIM5 is the physical symmetric MOSFET's model valid for RF-Analog circuit simulations.