High-Power-Density 0.25 µm Gate-Length AlGaN/GaN High-Electron-Mobility Transistors on Semi-Insulating 6H–SiC Substrates

AlGaN/GaN high-electron-mobility transistors (HEMTs) on 6H–SiC substrates with a 0.25 µm gate length and with different gate–drain spacings, L_GD, ranging from 1 to 7 µm have been fabricated for optimization of output power and investigation of trapping effects. A typical device with L_GD = 1 µm exhibited a maximum drain current density of 1400 mA/mm, a peak extrinsic transconductance (g_m) of 322 mS/mm, a device unity-gain cut-off frequency ( f_T) of 40.3 GHz, and a maximum frequency of oscillation ( f_MAX) of 85.3 GHz. Examination of small-signal RF and pulsed current–voltage measurements revealed that RF dispersion is closely related to the gate–drain spacing dimensions, indicating that large gate–drain spacing without the control of RF dispersion can adversely affect output power performance. The results were confirmed using load-pull performance measurements with better power and efficiency obtained for devices with shorter gate–drain separation. For a device with L_GD = 1 µm and a 0.25 µm gate-length, a state-of-the-art output power density of 6.7 W/mm at 18 GHz was obtained, which indicates the potential of GaN HEMTs achievable by control of RF current dispersion.