Trapping in an impurity (e.g. Fe, C) doped back barrier (BB) causes pronounced on-resistance (Ron) dispersion of GaN HEMTs. We demonstrate that the BB trapping is alleviated by increasing 2DEG density Nsh in the GaN channel (50% increased Nsh results in 30% less Delta mathrm{R} -{on}) and inserting an additional intrinsic AlGaN BB (100 nm AlGaN with 50% less Delta mathrm{R} -{on}). We propose a novel flat-AlGaN-BB-energy-band designing criterion for the AlGaN/C-GaN BB combination.