This paper reports the shallow trench isolation (STI)-induced mechanical stress-related Kink effect behaviour of the 40nm PD SOI NMOS device. As verified by the experimentally measured data and the 2D simulation results, the Kink effect behaviour in the saturation region occurs at a higher V D for the 40nm PD device with a smaller S/D length (SA) of 0.17μm as compared to the one with the SA of 1.7μm due to the higher body-source bandgap narrowing (BGN) effect on the parasitic bipolar device (BJT) from the higher STI-induced mechanical stress, offset by the impact ionization (II) enhanced by the BGN in the high electric field region near the drain.