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An experiment is designed to investigate the impact of surface traps on the breakdown voltage of passivated AlGaN/GaN HEMTs. The breakdown characteristics are measured after subjecting the devices to high-field off-state stress at various temperatures. The charged surface traps, which extend the depletion region in a channel, lead to a significant improvement of breakdown voltage. An increase of about 25% of breakdown voltage is observed after 3000%s-long stress. A characteristic time of about 300%s is obtained for the devices with 400%nm-thick SiN passivation. Analysis of the experimental results shows that although the surface traps are effectively suppressed by surface passivation, they still take effect when the devices are exposed to long-term high-field stress.