It has been shown that under accelerated stress below ap4 V, thin gate oxides are subject to progressive breakdown (BD), i.e., a gradual growth of the BD spot up to a destructive BD. This paper investigates the conduction mechanisms of the BD spot during the early stages of progressive BD through the measurement of the I-V characteristics using carrier separation. It is shown that a model with no free parameter based on the concept of cotunneling provides a good evaluation of the post-BD current. This model implies a physical microstructure, and its plausibility is compared to direct transmission electron microscopy (TEM) observations