Passivation Schemes for ScAlN-Barrier mm-Wave High Electron Mobility Transistors

This article presents improvements of large-signal RF power performance at Ka -band of gallium nitride high electron mobility transistors (HEMTs) utilizing a scandium aluminum nitride barrier. Three samples were fabricated and measured having different passivation schemes, including gallium nitride, silicon nitride, and aluminum gallium nitride. The pinning of the Fermi level against the valence band reduced the gain and output power density of the gallium nitride passivated device, resulting in 4.5 W/mm at a drain bias of 32 V. With silicon nitride alone as the passivation, dc/RF dispersion was reduced, and gain increased significantly, boosting the output power for this device to 8.0 W/mm at 28 V, though at the cost of a reduced breakdown voltage. Adding an epitaxial aluminum gallium nitride passivating layer increased the maximum bias voltage that the devices can operate at compared to devices with the silicon nitride passivation. When biased at 40 V, the output power of the devices reached 10.8 W/mm at 30 GHz. These devices show the importance of the passivation design to large-signal RF performance in scandium aluminum nitride-based HEMTs and will help guide further improvements.