The leakage current and the impact ionization effect causes a drawback for the performance of InAs/AlSb HEMTs due to the InAs channel with a very narrow band gap of 0.35 eV. In this paper, the conventional HEMT small-signal model was enhanced to characterize the RF behavior for InAs/AlSb HEMTs. The additional gate leakage current induced by the impact ionization was modeled by adding two resistances R-Gh1 and R-Gh2 shunting the C-gs-R-i and C-gd-R-j branches, respectively, and the ionized-drain current was characterized by an additional resistance R-mi parallel with the output resistance R-ds, meanwhile the influence of the impact ionization on the transconductance was modeled by an additional current source g(mi) controlled by V-gs. The additional inductance, evaluated as a function of f(omega, R), was introduced to characterize the frequency dependency of impact ionization by using the impact ionization effective rate 1/tau(i) and a new frequency response rate factor n, which guaranteed the enhanced model reliable for a wide frequency range. As a result, the enhanced model achieved good agreement with the measurements of the S-parameters and Y-parameters for a wide frequency range, moreover, the simulated results of the stability factor K, the cutoff frequency f(T), the maximum frequency of oscillation f(max), and the unilateral Mason's gain U were estimated to approach the experimental results with a high degree. (C) 2015 Elsevier Ltd. All rights reserved.