Anomalous Kink Effect in GaN High Electron Mobility Transistors

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.