This paper analyses the influence of the GaN and Si 3 N 4 passivation (or 'cap') layer on the top of the AlGaN barrier layer on the performance and reliability of Schottky barrier diodes with a gated edge termination (GET-SBDs). Both GaN cap and Si 3 N 4 cap devices show similar dc characteristics but a higher density of traps at the SiO 2 /GaN interface or/and an increase of the total dielectric constant in the access region result in higher R ON -dispersion in GaN cap devices. The leakage current at medium/low temperatures in both types of devices shows two low-voltage-independent activation energies, suggesting thermionic and field-emission processes to be responsible for the conduction. Furthermore, a voltage-dependent activation energy in the high-temperature range occurs from low voltages in the GaN cap devices and limits their breakdown voltage (V BD ). Time-dependent dielectric breakdown measurements show a tighter distribution in Si 3 N 4 cap devices (Weibull slope β = 3.3) compared to GaN cap devices (β = 1.8). Additional measurements in plasma-enhanced atomic layer deposition (PEALD)-Si 3 N 4 capacitors with different cap layers and TCAD simulations show an electric field distribution with a strong peak within the PEALD-Si 3 N 4 dielectric at the GET corner, which could accelerate the formation of a percolation path and provoke the device breakdown in GaN cap SBDs even at low-stress voltages.