Charge trapping and slow (10 s to > 1000 s) detrapping in AlGaN/GaN HEMTs designed for high breakdown voltage (> 1500 V) are studied to identify the impact of Al molefraction and passivation on trapping. Two different trapping components, TG1 (Ea = 0.62 eV) and TG2 (with negligible temperature dependence) in AlGaN dominate under gate bias stress in the off-state. Al0.15Ga0.85N shows much more vulnerability to trapping under gate stress in the absence of passivation than does AlGaN with a higher Al mole fraction. Under large drain bias, trapping is dominated by a much deeper trap TD. Detrapping under illumination by monochromatic light shows TD to have Ea ≈ 1.65 eV in Al0.26Ga0.74N and Ea ≈ 1.85 eV in Al0.15Ga0.85N. This is consistent with a transition from a deep state (Ec - 2.0 eV) in the AlGaN barrier to the 2DEG.