Time-Dependent Dielectric Breakdown Under AC Stress in GaN MIS-HEMTs.

We investigate time-dependent dielectric breakdown (TDDB) in AlGaN/GaN Metal-Insulator-Semiconductor High Electron Mobility Transistors (MIS-HEMTs) under forward bias AC stress which better emulates real-world operational conditions. To this end, we have performed TDDB experiments across a wide range of frequencies, temperatures, and recovery voltage levels. We find that TDDB under AC stress shows longer breakdown times than under DC stress and that this increase is more prominent with higher frequency, lower temperature, and more negative recovery voltage. We hypothesize that this is due to the dynamics of the gate stack in GaN MIS-HEMTs biased with a high positive gate voltage. Under these conditions, a second electron channel forms at the dielectric/AlGaN interface. This process is relatively slow as these electrons come from the 2DEG at the AlGaN/GaN interface and must overcome the energy barrier presented by the AlGaN. At the same gate voltage then, the electric field across the gate oxide is lower in magnitude under AC stress at high enough frequency than under DC stress explaining the obtained results.