NBTI and RTN time-dependent variability is described from a defect-centric perspective. It is shown that NBTI induced threshold voltage shift (ΔVTH) distribution is governed by a compound Exponential-Poisson process. Using the memoryless properties of Poisson statistics, it is shown that not only the stressed but also the relaxed fraction follows the Exponential-Poisson distribution. Moreover, the relaxed fraction is shown to be independent from the final remaining ΔVTH. Using the same statistical model, the separation of RTN induced ΔVTH from NBTI is shown which allows a more accurate estimation of the characteristic distribution parameters. Finally, it is shown that RTN is a wide-sense stationary noise source of which the autocorrelation can be extracted from analysis of NBTI relaxation traces.