The mechanism for quasi-breakdown (QB) in thin gate oxides was studied using bipolar current stress and unipolar constant current stress. Continual bipolar current stressing on the gate oxide shows two distinct stages of QB-recoverable and unrecoverable QB. During the recoverable QB stage, the gate leakage current recovers to the stress-induced leakage current level upon application of a proper reverse bias. In contrast, no electrical recovery is observed within the unrecoverable QB stage. This stage is characterized by a higher gate leakage current than that of the recoverable QB stage and a very stable gate voltage during stressing. Carrier separation measurements further demonstrate that two different modes of conduction can occur during the recoverable QB stage. In the early stage, Fowler-Nordheim electron tunneling dominates the conduction mechanism although a small hole current is observed. With prolonged electrical stress, the hole direct tunneling current becomes dominant. Based on the aforementioned observations and the monitoring of the generation of oxide traps using the direct-current current-voltage technique, a QB model of positive hole trapping at the anode is proposed. (C) 2002 American Institute of Physics.