An enhanced small-signal model is introduced to model the influence of the impact ionization effect on the performance of InAs/A1Sb HFET,in which an optimized fitting function D(ωτi) in the form of least square approximation is proposed in order to further enhance the accuracy in modeling the frequency dependency of the impact ionization effect.The enhanced model with D(ωτi) can accurately characterize the key S parameters of InAs/A1Sb HFET in a wide frequency range with a very low error function EF.It is demonstrated that the new fitting function D(ωτi) is helpful in further improving the modeling accuracy degree.