Abstract: An improved model for AlGaN/GaN HEMTs including Kink effect was presented in this paper. This large signal model was constructed based on the symbolically defined device (SDD) form of advanced design system (ADS). The improved <i<I-V</i< expression was proposed to complete nonlinear fitting accurately by contrasting the measure results of the on-wafer AlGaN/GaN HEMT with two gate fingers each being 90 nm long and 40 μm wide. The model can accurately fit the 0~110 GHz <i<S</i<-parameter and DC characteristics of the device. The convergence of the model is great during the harmonic balance simulation. So this modeling method can be applied to millimeter-wave GaN HEMTs.