The influence of gate structure on AlN/GaN/SiC HEMT (High electron mobility transistor) is analyzed in this paper. The design and comparison of a T-gate HEMT with a gate footprint of 100 nm and a rectangular gate HEMT with a gate length (L_G) of 200 nm is performed. The T-gate device was found to have better RF/DC performance with the highest G_M of 197.4 mS/mm, the greatest I_D of 0.674 A/mm, and the maximum f_T of 84.42 GHz. Moreover, by doping the Al_0.08Ga_0.92N buffer layer with Fe-impurities, the performance of the HEMTs is examined. In both HEMTs, the performance of the Fe-doped and undoped buffers is compared. From the comparison, the Fe-AlGaN buffer performed better in both cases. Besides, from the breakdown simulation results, it is observed that rectangular gate HEMT achieved the highest off- state breakdown voltage (V_BR) of 75.93 V. The V_BR recorded by the T-gate device was 63.82 V. These findings demonstrate that T-shaped HEMTs have significantly better RF/DC behavior than conventional HEMTs, making them the preferred choice for high-frequency applications including satellite, RADAR, and wireless communications, etc.

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