Trap induced negative differential conductance and back-gated charge redistribution in AlGaN/GaN power devices

Negative differential conductance (NDC) seen in the drain current saturation region is typically attributed to self-heating effects in GaN HEMTs. In this paper, it is demonstrated that NDC is due to trap states in the buffer resulting from poor carrier confinement in the channel during saturation which leads to stray electrons exiting the channel and becoming trapped. Substrate bias testing has been used to confirm that severe current collapse occurs when those electrons are forced to the surface, and that NDC occurs when the carriers are forced to the buffer. In addition to this, pulsed I-V characterization is performed among other tests to isolate the cause of NDC in the saturation region, which effectively eliminated trapping mechanisms that lead to gate- and drain-lag from contributing to NDC. To the best of our knowledge, we are the first to report NDC not as a result of self-heating, but due to trapping within the buffer.