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High-voltage (> 400 V) GaN high-electron mobility transistors were fabricated using two types of heterostructures with different buffer layer structures. The buffer layer structure affected the crystal defect density in grown AlGaN/GaN heterostructure. The static on-resistance under low applied voltage was independent of the buffer layer structure because it has no influence on the 2-D electron-gas density. On the other hand, the drain leakage current through the grown layers and the dynamic on-resistance increase caused by the current collapse phenomena depended on the buffer layer structure. The leakage current was reduced by the AlN/n-GaN/AlN layers because of the potential barrier at the AlN/n-GaN interface and no-depletion of the n-GaN layer. In addition, the experimental results showed that the dynamic on-resistance was increased with the edge dislocation density and was not influenced by the screw dislocation density. From these results, it can be expected that edge dislocation is related to the electron trapping center, which must be reduced to suppress the current collapse phenomena.