In this letter, we demonstrate a general model to reduce the reverse leakage current (I-R) in high-voltage AlGaN/GaN Schottky diodes (SBDs) by engineering the pinchoff voltage (V-p) of their field plates (FPs). The maximum voltage drop at the Schottky junction (V-SCH) in the OFF state can be significantly decreased by reducing vertical bar V-p vertical bar, which leads to a drastically diminished I-R. We used a tri-gate architecture as means to control V-p and, thus, I-R, as it offers great flexibility to engineer V-p compared with conventional schemes. vertical bar V-p vertical bar of SBDs with tri-gate FPs was reduced by decreasing the width of the nanowires, which led to a very small I-R, below 10 nA/mm under reverse biases up to 500 V, and an increase of over 800 V in soft breakdown voltage (V-BR) at 1 mu A/mm. These results reveal the importance of V-p in reducing I-R for SBDs, and unveil the potential of tri-gate structures as FPs for power devices.