The effects of plasma enhanced vapor deposited silicon nitride (SiNx) passivation layer thickness and the spacing between the contact windows openings in the SiNx layer on the isolation-blocking voltage of nitrogen ion implanted AlGaN/GaN high electron mobility transistors were studied. The isolation-blocking voltage was proportional to the thickness of the SiNx passivation layer. Early breakdown was observed for the samples without thick enough SiNx due to surface breakdown. The device was permanently damaged after the occurrence of this early breakdown. The dependence of the isolation-blocking voltage on the SiNx thickness was also modeled and the general trends of the simulated results were in good agreement with the experiment data. The effect of rf power used for depositing the SiNx layer on the isolation-blocking voltage was also studied. Ion bombardments during the SiNx deposition could cause the reduction of breakdown voltage. By employing optimized SiNx passivation conditions, a saturation drain current and a drain breakdown voltage of 300 mA/mm and 1000 V, respectively, for HEMTs with gate dimension of 1×200 μm2 and gate to drain distance of 37.5 μm were achieved.