Comparative studies of normally-off Al_0.26Ga_0.74N/AlN/GaN/Si high electron mobility transistors with different gate structures

Systematic designs to achieve normally-off operation and improved device performance for Al_0.26Ga_0.74N/AlN/GaN high electron mobility transistors (HEMTs) grown on a Si substrate are investigated in this work. The step-by-step approach includes: (1) devising a thin AlGaN/AlN composite barrier, (2) introducing fluoride ions within the active region by using CF₄ plasma treatment, (3) growing the Al₂O₃ oxide passivation layers within gate-drain/source regions by using a cost-effective ozone water oxidization technique, and (4) integrating a metal-oxide-semiconductor gate (MOS-gate) design with high-k Al₂O₃ gate dielectric. Devices with four different evolutionary gate structures have been compared and studied. Variations of threshold voltage (V_th), Hooge coefficients (α_H), maximum drain-source current density (I_{DS, max}), maximum extrinsic transconductance (g_{m, max}), gate-voltage swing (GVS) linearity, two-terminal gate-drain breakdown/turn-on voltages (BV_GD/V_on), on/off current ratio (I_on/I_off), and high-temperature characteristics up to 450 K are also investigated.