Negative bias temperature instability (NBTI) and channel hot carrier (CHC) are the leading reliability concerns for nanoscale transistors. The de facto modeling method to analyze CHC is based on substrate current I _sub , which becomes increasingly problematic with technology scaling as various leakage components dominate I _sub . In this paper, we present a unified approach that directly predicts the change of key transistor parameters under various process and design conditions for both NBTI and CHC effects. Using the general reaction-diffusion model and the concept of surface potential, the proposed method continuously captures the performance degradation across subthreshold and strong inversion regions. Models are comprehensively verified with an industrial 65-nm technology. By benchmarking the prediction of circuit performance degradation with the measured ring oscillator data and simulations of an amplifier, we demonstrate that the proposed method very well predicts the degradation. For 65-nm technology, NBTI is the dominant reliability concern, and the impact of CHC on circuit performance is relatively small.