“…The basic structure of surface passive layer is illustrated in the Figure 6. Many different insulation materials have been explored as the surface passivation layer for AlGaN/GaN HEMTs, including SiO 2 [93,99], Si 3 N 4 [100], ZrO 2 [94,101], HfO 2 [95,96], Ga 2 O 3 [43,102], AlN [103][104][105][106], Sc 2 O 3 [107], TiO 2 [108], ZnO 2 [109], NiO [110], Ta 2 O5 [111], Al 2 O 3 [112][113][114], AlON [43] and so forth. In addition to the surface passivation, dielectric-free passivation technologies have also been proposed and demonstrated, for example, oxygen plasma oxidization [115,116], ozone oxidization [117], chemical oxidization [118], SiH 4 treatment [119] and so forth.…”