Two different materials for double passivation layers have been implemented to an AlGaN/GaN high electron mobility transistor on Si (111) substrate and the improved DC properties are demonstrated. Si3N4 and SiO2 passivation materials are deposited on the gamma gate upper and bottom layers by plasma enhanced chemical vapor deposition. The gamma shape gate can be made by selectively accurate Si3N4 or SiO2 first passivation dry etching with wet etching. The second passivation on gamma gate effectively increases the DC properties. The effects of DC properties of Si3N4 or SiO2 single passivation and Si3N4/Si3N4 or SiO2/SiO2 double passivations are compared. The Si3N4/Si3N4 double passivation shows the maximum saturation current density and the peak extrinsic transconductance which increases up to 72% and 18%, respectively, more than Si3N4 single passivation and also up to 18% and 5% than SiO2/SiO2 double passivation. (C) 2012 Elsevier B.V. All rights reserved.