A Tall Gate Stem GaN HEMT With Improved Power Density and Efficiency at Ka-Band

In this letter, AlGaN/GaN HEMTs with tall-gate-stem structures were realized to improve the power performance of Ka-band devices, and a film thinning process is adopted in the fabrication process to reduce the parasitic capacitance caused by the thick silicon nitride film. According to the S-parameter measurement results, devices owning a tall-gate-stem structure and undergoing the film thinning process have higher cut-off frequency (fT) and maximum oscillation frequency (fmax) values with lower extracted parasitic capacitance. For the load-pull measurement result, the AlGaN/GaN HEMT with a tall gate stem has improved output power density (Pout) and power added efficiency (PAE) at Ka-band. The device with the elevated stem shows a steady-state current density of 883 mA/mm and a maximum transconductance of 323 mS/mm at 20 V bias, and it achieves
${\mathrm{ f}}_{\mathrm{ T}}$
of 39.5 GHz,
${\mathrm{ f}}_{\max }$
of 112.9 GHz with the maximum PAE of 24.6% and the maximum
${\mathrm{ P}}_{\mathrm{ out}}$
of 6.6 W/mm at 38 GHz.