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We investigated the limitations of the field plate (FP) effect on breakdown voltage VBD that is due to the silicon substrate in AlGaN/GaN/AlGaN double heterostructures field-effect transistors. In our previous work, we showed that in devices with large gate-drain distance (LGD > 8 μm), the breakdown voltage does not linearly increase with LGD because of a double leakage path between the silicon substrate and the metal contacts, which makes the device break at the silicon interface. In this paper, we showed that the effect of the FP for such large LGD is not significant because the breakdown is still dominated by the silicon substrate. The increase in VBD due to the FP is significant only for devices with small gate-drain distances (LGD <; 8 μm). Indeed we show that for such small LGD the increase in the breakdown voltage is more than double, whereas for larger LGD, this is only about 10%. Simulations of AlGaN/GaN/AlGaN devices for small LGD are carried out with different FP lengths and passivation thickness in order to study the electric field distribution.