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In this letter, AlGaN/GaN HEMTs with tall-gate-stem structures were realized to improve the power performance of Ka-band devices, and a film thinning process is adopted in the fabrication process to reduce the parasitic capacitance caused by the thick silicon nitride film. According to the S-parameter measurement results, devices owning a tall-gate-stem structure and undergoing the film thinning process have higher cut-off frequency (fT) and maximum oscillation frequency (fmax) values with lower extracted parasitic capacitance. For the load-pull measurement result, the AlGaN/GaN HEMT with a tall gate stem has improved output power density (Pout) and power added efficiency (PAE) at Ka-band. The device with the elevated stem shows a steady-state current density of 883 mA/mm and a maximum transconductance of 323 mS/mm at 20 V bias, and it achieves <inline-formula> <tex-math notation="LaTeX">${\mathrm{ f}}_{\mathrm{ T}}$ </tex-math></inline-formula> of 39.5 GHz, <inline-formula> <tex-math notation="LaTeX">${\mathrm{ f}}_{\max }$ </tex-math></inline-formula> of 112.9 GHz with the maximum PAE of 24.6&#x0025; and the maximum <inline-formula> <tex-math notation="LaTeX">${\mathrm{ P}}_{\mathrm{ out}}$ </tex-math></inline-formula> of 6.6 W/mm at 38 GHz.