We demonstrate the excellent performance of a 140 W AlGaN/GaN HEMT in the C-band, which is passivated by a Cat-CVD SiN film. The OF interface trap density of the AlGaN surface passivated by Cat-CVD film after NH3 treatment is 3 x 10(12) cm(-2), which is the smallest of investigated deposition techniques. The lowest interface trap density achieved by the Cat-CVD technique makes it possible to operate the AlGaN/GaN HEMT in the C-band. We clarify that the Cat-CVD technique is necessary for developing future amplifiers. (C) 2007 Elsevier B.V. All rights reserved.