This study proposes the investigation of using the (NH 4) 2 S x solution to form the AlGaN surface passivation on the AlGaN/GaN high electron mobility transistors (HEMTs). Both treatment schemes are implemented on separate pieces of the same HEMT wafer, including (NH 4) 2 S x pregate and post-gate metal treatments. Temperature-dependent characteristics of the HEMTs are also studied. Experimental results demonstrate that by the surface treatment prior to metal, the performance of the studied HEMTs can be improved, including thermal stability, dc and high-frequency characteristics. Furthermore, the interface state density (D it) of the studied HEMT is studied by the subthreshold slope method. To the best of our knowledge, this is the first report on comparison of AlGaN/AlN/GaN HEMTs with pre-gate and post-gate metal treatments.