AlN/GaN metal insulator semiconductor field effect transistor structures (MISFETs) with and without in situ SiNx were investigated. The in situ SiNx passivation layer was deposited using silane and ammonia immediately after the AlN/GaN heterostructure growth by metal organic vapor phase epitaxy. Superior Ohmic quality and improvement of the dc and rf characteristics were obtained with devices made on passivated layers in comparison with unpassivated devices. This enhancement of electrical characteristics is attributed to the fact that the in situ SiNx suppresses the depletion of the two-dimensional electron gas (2DEG) and air exposure while processing. The studies of the in situ SiNx quality and its implication to the 2DEG conducted with Hall, transmission line measurements, capacitance-voltage (C-V) measurements, and photoluminescence measurements showed that the electrical and optical characteristics of AlN MISFET structures were substantially degraded or changed after SF6 plasma treatment.