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We report small- and large-signal performances of 140-nm gatelength field-plated GaN HEMTs at Ka-band frequencies, in which the GaN HEMTs were fabricated with n+ source contact ledge. The parasitic channel resistance is reduced by ~ 50%, whereas the peak extrinsic transconductance is improved by 20% from 370 to 445 mS/mm. The GaN HEMTs with n+ source ledge exhibit improvement of maximum stable gain by at least 0.7 dB over reference devices without n+ ledge. At 30 GHz, CW output power density of 10 W/mm is measured with peak PAE of 40% and associated gain of 8.4 dB at Vds = 42 V. At Vds = 30 V, the output power density is measured as 7.3 W/mm with peak PAE of 50%, peak DE of 58%, and associated gain of 8.5 dB. The best PAE was measured as 55% at 5 W/mm at 30, 33, and 36 GHz, where the associated gains were 7.9, 7.6, and 8.2 dB, respectively.