In this work， high-efficiency AlN/GaN metal-insulator-semiconductor high electron mobility transistors （MIS-HEMTs） have been fabricated for millimeter wave applications. A 5-nm SiN_x insulator is grown in-situ as the gate insulator by metal-organic chemical vapor deposition （MOCVD）， contributing to remarkably suppressed gate leakage， interface state density and current collapse. The fabricated MIS-HEMTs exhibit a maximum drain current of 2.2 A/mm at V_GS=2 V， an extrinsic peak G_m of 509 mS/mm， and a reverse Schottky gate leakage current of 4.7×10^-6 A/mm when V_GS = -30 V. Based on a 0.15 um T-shaped gate technology， an f_T of 98 GHz and f_MAX of 165 GHz were obtained on the SiN/AlN/GaN MIS-HEMTs. Large signal measurement shows that， in a continuous-wave mode， the MIS-HEMTs deliver an output power density （P_out） of 2.3 W/mm associated with a power-added efficiency （PAE） of 45.2 % at 40 GHz， and a P_out （PAE） of 5.2 W/mm （42.2%） when V_DS was further increased to 15 V.