Existing literature indicates that there are two major mechanisms involved in the Time Dependent Dielectric Breakdown (TDDB) of silicon dioxide, and each mechanism dominates under different stress conditions. We suggest that the thermochemical (linear E) model and the hole-induced (1/E) model can be unified in one model. Based on the unified model, a wide range of TDDB data from different sources were examined and shown to behave consistently. Temperature and stress field dependencies are treated together in the model so that the lifetime is a single-valued function of temperature and field. The criterion for screen/ramp breakdown test is also discussed with the model. Furthermore, the unified model accounts for the effect of gross defects, which limit the oxide reliability in real IC's. The model is be used to predict the 10-year lifetime breakdown field or acceptable oxide thickness for given voltage, and results suggest that further refinements for thin oxide (<nm) are necessary.