The construction and validation of a quasi-static nonlinear microwave model for FinFETs are investigated. A very good agreement between model simulations and measurements is obtained under different DC bias points, input power levels, fundamental frequencies up to 5 GHz, and device geometries. Since the intrinsic part of the FET model is based on the quasi-static approximation, the goal of this work is to analyze in detail the limitations of this assumption by examining the intrinsic admittance parameters versus the frequency.