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Due to the low mobility and wide bandgap characteristics of the undoped AlGaN layer used in the conventional AlGaN-GaN HEMT as a cap layer, the RF performance of this device will be limited by its high contact resistance and high knee voltage. In this letter, we propose using the n+-GaN cap layer and the selective gate recess etching technology on the AlGaN-GaN HEMTs fabrication. With this n+-GaN instead of the undoped AlGaN as a cap layer, the device contact resistance is reduced from 1.0 to 0.4 Ω·mm. The 0.3 μm gate-length device demonstrates an Ids,max of 1.1 A/mm, a gm,max of 220 mS/mm, an fT of 43 GHz, an fmax of 68 GHz, and an output power density of 4 W/mm at 2.4 GHz.