AlGaN/GaN high electron mobility transistors (HEMTs) are excellent candidates for high power and high frequency applications at room and elevated temperatures due to their superior material properties. As a result of improved material growth and processing technologies, microwave power densities have been demonstrated that are five to ten times greater than that of corresponding GaAs-based devices. Though GaN HEMTs using field plate have demonstrated power densities as high as 32W/mm at 4 GHz, however to date, there have been only few reports on field-plated devices up to X band [1,2]. In this paper, we present record power performance of AlGaN/GaN HEMTs on 6H-SiC substrates at 18 GHz. A CW output power density of 9.1 W/mm with a gain of 5.8 dB and power added efficiency of 23.7 % were achieved. The AlGaN HEMT structure used in the present study was grown on 6H-SiC substrates by metal organic chemical vapor deposition (MOCVD). The epilayer consists of an AlN buffer, 1.5 μm undoped GaN, and a 20 nm Al .30Ga .70N barrier layer. Average sheet resistance across the as-grown wafer was 380 Ω/sq as measured by Leighton. Device fabrication started with mesa isolation using Cl 2/Ar plasma in an inductively-coupled-plasma reactive ion etch (ICP-RIE) system. Alloyed ohmic contacts of Ti/Al/Mo/Au were formed at 850°C with a low contact resistance of ∼ 0.15 ohm-mm. The source-drain spacing for these devices was 2.7 μm. Next, silicon nitride was deposited using PECVD system. Then 0.25 μm gate-footprints were patterned using e-beam lithography and etched through the silicon nitride film in a RIE system. The distance between the gate-footprint and source contact was 0.8 μm for all transistors. Finally, Ni/Au (300/2500 Å) gamma-gates with different side-lobe lengths on the drain side were deposited by e-beam evaporation. Three side-lobe (field-plate) lengths were designed: 0.9, 1.2, and 1.5 μm. The devices had a total gate width of 100 μm. On-wafer DC measurements were performed using an HP4145B semiconductor parameter analyzer. Devices with different lengths of field plates had similar dc characteristics. Figure 1 shows a typical drain current-voltage (I D-V DS) characteristics for the device with a field plate length (L FP) of 1.5 μm. The gate was biased from -5 V to 2 V in a step of 1 V. The devices exhibited a maximum drain current density (I D,max) of 1.42 A/mm at a gate bias of 2 V and a drain bias of 9 V. The DC transfer characteristics for this device are shown in Fig. 2. The drain was biased at 7 V. A peak extrinsic transconductance (g m of 437 mS/mm was measured at V gs = -3.2 V. The high value of g m is attributed to the thin AlGaN barrier layer and the low contact resistance. On-wafer small signal RF measurements were carried out using a Cascade Microtech Probe and an HP8510C network analyzer. With the increase of field plate length from 0.9 to 1.5 μm, the cut-off frequency (f T) decreased from 50 to 41 GHz while the maximum frequency of oscillation (f max) degraded from 81 to 63 GHz. This is attributed to an increase in the gate-drain capacitance. Figure 3 shows the small signal RF performance of the device with the field plate length (L FP) of 1.5 μm. Large signal CW measurements at 18 GHz were performed using a Focus Microwave automatic load pull system. The data were taken on-wafer at room temperature without any thermal management. At a drain bias of 40 V, power densities of 5.4, 6.4 and 7.3 W/mm were measured for devices with L FP of 0.9, 1.2 and 1.5 μm, respectively. Large signal performance of the device with LFP of 1.5 μm at a drain bias of 55 V is shown in Fig. 4. The device has an output power of 29.57 dBm corresponding to 9.1 W/mm with an associated gain of 5.8 dB and PAE of 23.7 %. In summary, we have presented the CW power performance of 0.25 μm gate-length AlGaN/GaN HEMTs with field plates at 18 GHz. These results demonstrate the exceptional potential of these devices for hieh oower aonlications bevond X band.