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Comprehensive RF stress-test campaign has been performed over AlGaN/GaN high-electron mobility transistor employing different GaN buffer designs, including unintentional doping, carbon doping and iron doping. No signature of gate-edge degradation has been found, and good correlation emerges between the buffer composition, subthreshold leakage current, and permanent degradation of the RF performance. The degradation mechanism, more pronounced in devices with parasitic buffer conductivity, involves the generation of additional deep trap states, the worsening of the dynamic current collapse, and the subsequent degradation of RF output power

An InAlN/AlN/GaN HEMT with Au‐free Ta‐based ohmic contacts and a high‐quality PECVD SiN pas‐sivation is reported. The ohmic contacts were annealed at 550 °C, resulting in a contact resistance of 0.64 Ωmm. The gate length was 50 nm. The device performance and the process were evaluated by performing DC‐, pulsed IV‐, RF‐, and load‐pull measurements. It was observed that current slump was effectively mitigated by the passivation layer. The DC channel current density increased by 71 % to 1170 mA/mm at the knee of the IV curve, and the transconductance increased from 382 to 477 mS/mm after passivation. At the same time the gate leakage increased, and the extrinsic fmax decreased from 207 to 140 GHz. Output powers of 4.1 and 3.5 W/mm were measured after passivation at 31 and 40 GHz, respectively. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)