Al 2 O 3 /AlGaN metal-oxide-semiconductor capacitors show a hysteretic behavior in their capacitance vs voltage characteristics, often attributed to near-interface traps deriving from defects within the oxide layer. The origin as well as the structural/electronic properties of such defects are still strongly debated in the literature. Here, we use ab initio molecular dynamics and the climbing-image nudged elastic band method to show that aluminum Frenkel defects give rise to bistable trap states in disordered and stoichiometric Al2O3. Based on these results, we propose a calibrated polaron model representing a distribution of individually interacting energy levels with an internal reconfiguration mode and coupled to continuous bands of carriers to explain the hysteresis mechanism in Al2O3/AlGaN capacitors.