The effects of thin SiNx deposition on AlGaN/GaN heterostructure field-effect transistors were systematically studied by comparing their electrical and device characteristics. Two aspects of the thin SiNx film deposition were investigated: (i) the increase in two-dimensional electron gas (2DEG) density at the heterointerface and (ii) its capability as a gate insulating layer. Three different SiNx deposition methods were studied: catalytic chemical vapor deposition (Cat-CVD), metalorganic chemical vapor deposition (MOCVD), and plasma enhanced chemical vapor deposition (PECVD). A large increase in 2DEG density was obtained after SiNx deposition for all methods. The devices with MOCVD SiNx gate insulator showed a larger gate leakage current than those with the Cat-CVD and PECVD SiNx, implying that a thinning of the AlGaN surface barrier occurred due to Si diffusion into the AlGaN barrier during the high-temperature MOCVD process.