The influence of mesa edge capacitance on the frequency characteristics of AlGaN/GaN HEMTs with 90 nm gate length was studied in this paper. To extract mesa edge capacitances, a small-signal equivalent circuit model considering mesa edge capacitances was provided. Based on the model, the intrinsic gate capacitances of AlGaN/GaN HEMTs with 2 x 20 mu m, 2 x 30 mu m, 2 x 40 mu m, and 2 x 50 mu m gate widths were extracted, respectively. Through linear fitting along gate width for the extracted results and simulations, 8.06 fF/mu m(2) of mesa edge capacitances at V-gs=-4.5 V and V-ds = 8 V in the devices with 2 x 20 mu m gate width was obtained, which can be about 33.2% of the total gate capacitance. Mesa edge capacitances results in a significant drop of current-gain cut-off frequency (f(T)), and the effect is more serious in the shorter gate length devices. (C) 2016 Elsevier Ltd. All rights reserved.