The impact of surface treatment of 5 nm in situ SiN passivation on the performance of InAlGaN/GaN high electron mobility transistors is analyzed. After operation at 600 degrees C, no device degradation is observed, confirming the thermal stability of the passivation layer. The in situ SiN, grown by metalorganic chemical vapor deposition, is treated by first Ar ion sputter cleaning, intentional oxidation in water and second cleaning. The results of current collapse characterization show that the SiN surface contains a humidity related trap density of about 1.04 x 10(12) cm(-2), which can be reduced to 0.21 x 10(12) cm(-2) by Ar sputter cleaning. The surface. cleaning allows for deposition of further layers without degradation of the device properties. In this study, 25 nm SiN is deposited subsequently by conventional plasma enhanced chemical vapor deposition, resulting in the same device performance as the devices processed with 30 nm in situ SiN. (C) 2013 Elsevier Ltd. All rights reserved.