Impact of Source Resistance on Linearity of AlGaN/GaN HEMTs at Ka-Band

The impact of the source resistance (R_S) on the linearity of AlGaN/GaN high electron mobility transistors (HEMTs) measured by the two-tone test is investigated in this study. The theoretical analysis of the third-order intermodulation (IM3) and the third-order intercept point (IP3) of the devices was performed based on the equivalent circuit approach and the polynomial curve fitting technique. The results revealed that the suppression of IM3 level and thus the improvement of IP3 were achieved for the device with a larger R_S. For the experimental verifications, three types of AlGaN/GaN HEMTs were fabricated with different layout structures and air-bridge architectures corresponding to different R_S values, which were then extracted by the cold-FET method. It is concluded that a device with a shorter length of middle source pad (L_MS) and a side-type air-bridge architecture tends to have a larger R_S value. In this study, the 4 × 50-20 device with L_MS of 20 um and a side-type air-bridge architecture demonstrated the best-measured linearity performance at 30 GHz due to the largest R_S.