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An impact of the different voltage ramps (from 25mV/s to 20V/s) in off-state regime for HFET and MOS-HFET was experimentally investigated. Because of the temperature in the channel drops at low voltage steps, density of temperature-active interface states and states in the GaN buffer near the channel is reduced, and therefore the IDS,off-state decreases. A positive temperature coeficient is attended. An another behaviour for MOS-HFET for fast traps in comparison to HFET was observed. A smaller temperature-dependence of fast traps in comparison to slow traps can be responsible for this effect. The same tendency of IDS,off.state for all devices were found. The MOSHFET exhibited no change of drain current IDS,off-state for 5 hours, but for the HFET the value was more than 500 times higher.