High-voltage gallium nitride Schottky barrier diodes (SBDs) suffer from large off-state leakage current, which further degrades during operation at high temperatures and limits the device blocking capabilities. The key to achieving low off-state leakage is to protect the Schottky barrier from the high electric field, which is challenging by employing conventional field plate structures due to their large pinch-off voltage. In this work, we propose a simple AlGaN/GaN SBD architecture based on a p-GaN cap layer to achieve excellent off-state performance with a very low leakage current. By properly designing the AlGaN barrier and p-GaN cap, the pinch off-voltage of the p-GaN field plate is carefully controlled and the voltage drop over the Schottky junction is effectively reduced. In addition, a large carrier concentration in the access region is achieved, leading to a reduced sheet resistance. This results in good on-state performance along with a very low leakage current of ∼1 nA/mm at 400 V, which is maintained well below 100 nA/mm up to elevated temperatures of 150 °C. Moreover, the proposed architecture shares the well-established fabrication process of commercial p-GaN HEMTs and, thus, represents a promising and viable solution for future GaN diodes.