Novel T-Shaped Gate With Air Gap for AlGaN/GaN HEMTs on Silicon With High Johnson’s Figures of Merit

We present high-performance AlGaN/GaN high-electron-mobility transistors (HEMTs) fabricated on a silicon substrate featuring a novel T-shaped gate. The incorporation of the air gap between the gate and the low- k passivation layer effectively reduces the gate capacitance and thus improves the RF characteristics of the device. The prepared device with a gate length of 180 nm has a unit current gain cutoff frequency ( f <tex-math notation="LaTeX">_T</tex-math> ) of 55.2 GHz, a maximum oscillation frequency ( <tex-math notation="LaTeX">f_MAX</tex-math> ) of 73.7 GHz, and a three-terminal OFF-state breakdown voltage (B V <tex-math notation="LaTeX">_gd</tex-math> ) of 143 V at the gate–drain distance of 2.2 <tex-math notation="LaTeX">μ</tex-math> m. The estimated Johnson's figure of merit (J-FOM <tex-math notation="LaTeX">$=$</tex-math> B V <tex-math notation="LaTeX">_gd_T</tex-math> ) is 7.9 THz <tex-math notation="LaTeX">·</tex-math> V, representing a significant 30% improvement in comparison to the device with only stacked passivation layers. Besides, the J-FOM surpasses that of the single HfO <tex-math notation="LaTeX">_2</tex-math> passivation and single SiO <tex-math notation="LaTeX">_2</tex-math> passivation devices by 64.6% and 58%, respectively. These results clearly indicate that the AlGaN/GaN HEMTs with novel T-shaped gate design have great potential for radio frequency (RF) power device applications.