This dissertation details the synthesis, characterization, and application of low-pressure chemical vapor deposited (LPCVD) Aluminum Silicon Nitride (Al x Siy Nz ) dielectrics to AlGaN/GaN based high electron mobility transistors (HEMTs). A detailed study of the effect on A fabrication process for such devices utilizing the Al x Siy Nz passivation is presented, and their performance at frequencies ranging from DC to the microwave is compared to devices utilizing more traditional Siy Nz films for the same purpose. As AlGaN/GaN HEMT technology has matured, much focus has been placed on the passivation dielectric itself, as many parasitic device mechanisms are directly influenced by it. Metrics such as breakdown voltage, efficiency, and high-frequency dispersion are all greatly affected by the type of passivation applied. In this work the application of LPCVD Al x Siy Nz films as AlGaN/GaN HEMT passivants is presented and shown to yield superior devices to ones passivated with similarly deposited Siy Nz dielectrics. Breakdown voltage, power added efficiency (PAE), output power density were all measured to be superior on the Al x Siy Nz passivated devices when measured under large-signal conditions at 10 GHz.