High Performance Enhancement-Mode Gate-Recessed Al₂O₃/AlGaN/GaN MIS-HEMTs for X-band Applications

In this letter, enhancement-mode (E-mode) Al₂O₃/AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) with a gate-length (L_g) of 0.25 μm are fabricated using gate recess and ALD-Al₂O₃ process on a sapphire substrate. The fabricated E-mode MIS-HEMTs exhibit a threshold voltage of +2.2 V and a saturation drain current of 512.3 mA/mm at V_GS=8 V. The interface trap state density (D_it) extracted by frequency/temperature-dependent capacitance-voltage (C-V) measurement between Al₂O₃ and AlGaN decreases from 8.50×10¹² cm^-2eV^-1 at the energy of (E_C-0.35) eV to 9.73×10¹¹ cm^-2eV^-1 at (E_C-0.65) eV. In addition, the fabricated devices show excellent RF performance which consists of a cut-off frequency (f_T) of 30.5 GHz and a maximum oscillation frequency (f_max) of 71.5 GHz. Large-signal power measurements at 8 GHz reveal a maximum saturated output power density of 1.7 W/mm with a corresponding PAE of 32.6% in continuous-wave mode. These results showed the gate-recessed E-mode MIS-HEMTs are candidates for X-band applications.