The current invention provides a method of determining the lifetime of a semiconductor device due to time dependent dielectric breakdown (TDDB). This method includes providing a plurality of samples of dielectric layer disposed as a gate dielectric layer of a MOS transistor, approximating a source/drain current density distribution as a first function of voltage applied on the samples, approximating a substrate current density distribution as a second function of voltage applied on the samples, approximating a dielectric layer lifetime distribution as a third function of source/drain current density and substrate current density in the samples, deriving, from the first, the second, and the third functions, an empirical model wherein a dielectric layer lifetime is a function of voltage applied thereon, and using the model to determine dielectric layer lifetime at a pre-determined operating gate voltage.