Considerable amount of works have been reported to achieve a high breakdown voltage of AlGaN/GaN heterostructure devices by employing additional process such as SiO2 passivation1,2, floating metal rings and Ni/Au Oxidation3. However, it should be point out that treatment of passivation layer of AlGaN/GaN heterostructure devices has been reported scarcely. As+ ion implantation on SiO2 passivation layer may be a simple and effective to reduce electric field strength to increase breakdown voltages. The cross-sectional view of the proposed AlGaN/GaN Schottky barrier diode is shown in Fig. 1. We fabricated conventional AlGaN/GaN Schottky barrier diode and passivated the device with SiO2 layer of 350 nm thick. Finally As+ ions were implanted on the SiO2 passivation layer. We measured the surface potential of the test samples with electric force microscopy (EFM) in order to verify that implanted As+ ions remained as positively charged ions in SiO2 layer after ion implantation. After ion implantation, 2 dimensional electron gas (2DEG) concentration was increased slightly from 8.28E12 /cm2 to 8.38E12 /cm2 so that the forward current was also increased slightly. Table shows the breakdown voltages of the SBDs before and after As+ ion implantation. After As+ 80 keV 1 × 1E14 atoms/cm2 implantation, the breakdown voltage increased considerably from 604 V to 1204 V due to the edge termination by implanted As+ ions. The reverse leakage current decreased from 80.3 uA/mm to 21.2 nA/mm due to the relaxation of electric field concentration by As+ ion implantation. We verified the electric field relaxation through 2D simulation. After As+ ion implantation, the depletion region curvature under the reverse biased condition became moderate so that the maximum electric field strength was decreased. As+ ion implantation method may be a simple and effective edge termination method for improving the breakdown voltage as well as the leakage current of the proposed AlGaN/GaN SBDs. Proposed AlGaN/GaN SBDs showed high breakdown voltage of 1204 V and low leakage current of 21.2 nA/mm without any considerable decrease of forward characteristics while that of conventional device was 604 V and 80.3 uA/mm, respectively.