“…Currently, the most matured GaN HEMTs are based on a AlGaN/GaN heterostructure [ 6 , 7 , 8 , 9 , 10 ]. More recently, Al-rich ultrathin sub-10 nm barrier heterostructures have received much attention for millimeter-wave applications [ 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 ]. This is because they can deliver significantly higher 2DEG sheet carrier density compared to AlGaN/GaN HEMTs while offering the possibility to highly scale the epitaxial structure as needed when using short gate lengths [ 23 , 24 , 25 ].…”