Influence of mesa edge capacitance on frequency behavior of millimeter-wave AlGaN/GaN HEMTs

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 × 20 μm, 2 × 30 μm, 2 × 40 μm, and 2 × 50 μm gate widths were extracted, respectively. Through linear fitting along gate width for the extracted results and simulations, 8.06 fF/μm2 of mesa edge capacitances at Vgs = −4.5 V and Vds = 8 V in the devices with 2 × 20 μm 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 (fT), and the effect is more serious in the shorter gate length devices.