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Metal-organic chemical vapor deposition-grown field-plated 0.25-μm gate-length AlGaN/GaN high-electron mobility transistors (HEMTs) with field-plate lengths of 0.5, 0.8, and 1.1 μm have been fabricated on 6H-SiC substrates. These 0.25-μm gate-length devices exhibited maximum drain current density of more than 1.4 A/mm and peak extrinsic transconductance of 437 mS/mm. No dependence of dc I-V as well as transfer characteristics on field-plate length was observed. With increase of field-plate length, degradation in the value of unity current gain frequency fT and maximum frequency of oscillation fmax was observed, but there is significant improvement in breakdown voltage and power densities. Also, at 18 GHz, a continuous-wave output power density of 9.1 W/mm with power added efficiency of 23.7% was obtained for device with field-plate length of 1.1 μm, yielding the highest reported power performance of AlGaN/GaN HEMTs at 18 GHz.