A new technique using WSiN film as a protective cap layer of the internal ohmic metallization scheme and the GaN surface was developed to improve the surface morphology of the contact of AlGaN/GaN high electron mobility transistors (HEMTs). After annealing, this layer was selectively removed by patterning and dry etching. Metal contact surfaces covered with WSiN preserved a good surface morphology and edge definition. Moreover, the devices have a saturation current of 1A/mm and a maximum transconductance of 235 mS/mm. When biased at 30V, the output power density is 5.8 W/mm at 2GHz. These results indicate a damage-free process for the smooth ohmic contacts formation.