AlGaN/GaN high-electron-mobility transistors (HEMTs) were successfully fabricated on high- resistivity (HR) Silicon substrate. Surface passivation effects on AlGaN/GaN HEMTs were studied using Si3N4 dielectric layer grown by plasma enhanced chemical vapor deposition. The device direct-current current-voltage (IDS-VDS) characteristics, pulsed IDS-VDS characteristics, microwave small-signal and large-signal characteristics were studied before and after passivation. An enhancement of drain current (ID) density and the extrinsic transconductance was observed for the devices with full Si3N4 passivation. An improvement of fT, fmax and device output power (Pout) was also observed after surface passivation. A new evaluation method implemented to identify the ID collapse related traps from AlGaN/GaN HEMTs. More than 80% of ID collapse suppression and 80% of increase in Pout were observed in AlGaN/GaN HEMTs by Si3N4 passivation. The Pout increase in AlGaN/GaN HEMTs by surface passivation is in good agreement with the ID collapse suppression in AlGaN/GaN HEMTs by surface passivation.