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Three different passivation layers (SiNx, MgO and Sc2O3) were examined for their effectiveness in mitigating surface-state-induced current collapse in AlGaN/GaN high electron mobility transistors (HEMTs). The plasma-enhanced chemical vapor deposited SiNx produced ∼80-85% recovery of the drain-source current, independent of whether SiH4/NH3 or SiD4/ND3 plasma chemistries were employed. Both the Sc2O3 and MgO produced essentially complete recovery of the current in GaN-cap HEMT structures and ∼80-95% recovery in AlGaN-cap structures. The Sc2O3 had superior long-term stability, with no change in HEMT behavior over 5 months aging.