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An anomalous kink effect has been observed in the room-temperature drain current ID versus drain voltage V DS characteristics of GaN high electron mobility transistors. The kink is originated by a buildup (at low V DS) and subsequent release (at high V DS) of negative charge, resulting in a shift of pinch-off voltage VP toward more negative voltages and in a sudden increase in ID. The kink is characterized by extremely long negative charge buildup times and by a nonmonotonic behavior as a function of photon energy under illumination. The presence of traps in the GaN buffer may explain both spectrally resolved photostimulation data and the slow negative charge buildup.