Field-plated AlGaN/GaN HEMTs with power density of 9.1 W/mm at 18 GHz

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.