Suppression of Dispersive Effects in AlGaN/GaN High-Electron-Mobility Transistors Using Bilayer SiNx Grown by Low Pressure Chemical Vapor Deposition

A bilayer SiN_x passivation scheme has been developed using low pressure chemical vapor deposition (LPCVD), which effectively suppresses the dispersive effects in AlGaN/GaN high-electron-mobility transistors (HEMTs) for microwave power operation. The bilayer LPCVD passivation is compared with in-situ SiN_x passivations by metal-organic chemical vapor deposition (MOCVD) and ex-situ SiN_x passivations by plasma-enhanced chemical vapor deposition (PECVD). The HEMTs were fabricated and characterized in terms of pulsed IV, transient drain current, and load pull. The devices passivated with in-situ MOCVD SiN_x or PECVD SiN_x exhibit significant current slump (~ 40% and knee-voltage walkout, while the bilayer LPCVD SiN_x passivated device shows negligible current slump (~6% and knee-voltage walkout. These characteristics are directly reflected in the large signal operation, where HEMTs with bilayer LPCVD SiN_x have the lowest dynamic ON-state resistance and highest output power (5.4 W/mm at 3 GHz).