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GaN-based high-electron mobility transistors with planar multiple grating field plates (MGFPs) for high-voltage operation are described. A synergy effect with additional electron channel confinement by using a heterojunction AlGaN back barrier (BB) is demonstrated. Suppression of the OFF-state subthreshold gate and drain leakage currents enables breakdown voltage enhancement over 700 V and a low ON-state resistance of 0.68 mΩ × cm2. Such devices have a minor tradeoff in ON-state resistance, lag factor, maximum oscillation frequency, and cutoff frequency. A systematic study of the MGFP design and the effect of Al composition in the BB is described. Physics-based device simulation results give insight into electric field distribution and charge carrier concentration, depending on the field plate design.