This paper investigates off-state current leakage in strained silicon NMOSFETs built on supercritical thickness strained silicon films. it describes a simple conceptual model for the off-state leakage: the leakage is caused by enhanced dopant diffusion near misfit dislocations. This paper studies the validity of this hypothesis using both DC IV measurements and photon emission microscopy measurements of NMOSFETs with several different supercritical strained silicon thickness values. It shows that the emission measurements of the off-state leakage and the gate length dependence of the off-state leakage can both be easily explained by the leakage model. It also demonstrates that the response to an applied gate or back-bias voltage of the IV characteristics and the light emission data can be understood within the context of the proposed conceptual model.