We have investigated the effects of surface passivation thickness on the electrical performance of Al-GaN/GaN HEMTs with slant field plates. It is found that the existence of the silicon nitride passivation layer helps to improve the DC characteristics of the devices in terms of the lower drain current collapse, higher maximum DC transconductance, and higher maximum drain current. RF wise, the devices with passivation suffer from lower fT and fMAX due to the increase in the gate capacitances. However, an improvement of RF output power was observed for devices with passivation layer thickness up to 300 nm.