Progressive breakdown in high-voltage GaN MIS-HEMTs

We have investigated the time-dependent dielectric breakdown (TDDB) characteristics of high-voltage AlGaN/GaN Metal-Insulator-Semiconductor High Electron Mobility Transistors (MIS-HEMTs). We focus in particular on the phenomenon known as progressive breakdown (PBD), marked by an onset of noise in the gate current IG during forward gate bias stress. We observe classic PBD behavior characterized by a rapid increase of IG noise during stress that takes place soon before hard breakdown (HBD). The onset of PBD also marks a change in the subthreshold characteristics of the transistor: the gate leakage increases above the measurement noise floor in a step-like fashion, with this additional leakage flowing out the source and/or drain terminals. After PBD, the subthreshold IG also shows a power law temperature dependence. The capacitance-voltage characteristics measured both before and after PBD confirm that device degradation does not occur at the AlGaN/GaN interface. All results are consistent with observations in silicon MOSFETs that support the percolation model of defects behind PBD and HBD. This gives hope that proper physical models suitable for lifetime estimation can be developed for TDDB in GaN MIS-HEMTs.