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The transient behavior of AlGaN-GaN MIS-HEMTs were studied by drain current deep level transient spectroscopy. Two electron traps were observed, one of which had similar activation energy to that of defect that was commonly observed in epitaxial GaN. We compared the results with those of AlGaN-GaN HEMTs. The hole-trap-like positive peaks in the DLTS, which were observed in the HEMTs, were not observed in the MIS-HEMTs. It has been pointed out that the positive peaks did not originate from change in hole trap population in the channel but reflected the change in the electron population in the surface states of the HEMT access regions. The gate insulator was effective to suppress not only the gate leakage current but also the surface-state-related signals.