Methodology for determining interface-state density in AlGaN/GaN MIS-HEMT structures using sub-bandgap illumination

In this work, we present a methodology for determining the interface-state density distribution in metal-insulator-semiconductor (MIS) AlGaN/GaN heterostructures using sub-bandgap illumination. First, we discuss the electronic transitions that occur during sub-bandgap illumination of MIS AlGaN/GaN heterostructures. In particular, we show how these transitions can induce a threshold-voltage shift in MIS AlGaN/GaN HEMT structures and, consequently, lead to a completely erroneous interpretation of the results. In this context, we experimentally demonstrate that the interface-state density distribution can be significantly overestimated due to, for example, free-hole generation resulting from electronic transitions via interface states at the AlGaN/insulator interface and/or deep levels in the GaN buffer layer. Finally, we provide guidelines for the accurate determination of the interface-state density in MIS AlGaN/GaN heterostructures using sub-bandgap illumination, including detailed measurement and analysis procedures.