This paper reports on the fabrication and characterization of gate-last self-aligned in situ SiN x /AlN/GaN MISHEMTs. The devices featured in situ grown SiN x by metal-organic chemical vapor deposition as a gate dielectric and for surface passivation. Selective source/drain regrowth was incorporated to reduce contact resistance. SiN x sidewall spacers and low-κ benzocyclobutene polymer (κ = 2.65) supporting layers were employed under the gate head to minimize the parasitic capacitance for high-frequency operation. The device with a gate length (L G ) of 0.23 μm exhibited a maximum drain current density (I DS ) exceeding 1600 mA/mm with a high ON/OFF ratio (I ON /I OFF ) of over 10 7 . The current gain cutoff frequency ( f T ) and maximum oscillation frequency ( f max ) were 55 and 86 GHz, respectively. In addition, the effect of temperature, from room temperature up to 550 K, on the dc and RF performances of the gate-last self-aligned MISHEMTs was studied.Index Terms-AlN/GaN, benzocyclobutene (BCB) planarization, gate-last self-aligned, high-temperature, in situ SiN x , MISHEMT, RF, source/drain (S/D) regrowth.