Prior work has shown that the OFF-state breakdown voltage, VBR, of AlGaN/GaN high electron mobility transistors (HEMTs) can be raised by just extending the drain end of the gate as a field plate (FP) or a high-k passivation over the AlGaN layer. However, the two techniques have not been compared, and the VBR for which their effectiveness was demonstrated was ≥120 V. High-k passivation avoids the lithography involved in realizing a FP, and the VBR of practical devices can go down to a few 10's of volts. Hence the present work compares the effectiveness of these techniques by TCAD simulations and establishes the following. At VBR of few 10's of Volts, the high-k passivation is much less effective than the FP, e.g. for a device with VBR = 28 V, a 0.7 μm long FP over 0.105 μm thick SiN doubles the VBR to 56 V, whereas even a 0.5 μm thick HfO2 passivation (εr = 20) has almost no effect on VBR. However, such differences between the two techniques reduce progressively as VBR is raised. The differences vanish for VBR ≥ 100 V, for which both techniques can improve the VBR by ≥ 2.5 times. Although focused primarily on breakdown, our work gives quick calculations showing that the high-k passivation has 5 – 10 % higher gate-drain capacitance than the gate-connected FP.