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A physical yet compact gate delay model is developed integrating short-channel effects and the Alpha-power law based timing model. This analytical approach accurately predicts both nominal delay and delay variability over a wide range of bias conditions, including sub-threshold. Excellent model scalability enables efficient mapping between process variations and delay variability at the circuit level. Based on this model, relative importance of physical effects on delay variability has been identified. While effective channel length variation is the leading source for variability at current 90nm node, performance variability is actually more sensitive to threshold variation at the sub-threshold region. Furthermore, this model is applied to investigate the limitation of low power design techniques in the presence of process variations, particularly dual Vth and L biasing. Due to excessive variability under low VDD, these techniques become ineffective.