In this study, the scheme of direct substrate potential measurement is used to explore the real-time substrate potential of the gate-grounded NMOS (GGNMOS). Through the parasitic capacitor between two pins of the packaged device, the coupled voltage can be transmitted to the substrate. It can be found that the substrate potential varies with location and pulse time evolution under transmission-line pulse (TLP) stress. Moreover, the substrate potential can propagate along the channel of the GGNMOS. Based on our experimental results, the substrate potential propagation can affect the turn-on of parasitic n-p-n bipolar in the GGNMOS. To simulate this dynamic behavior, the equivalent RC ladder circuit in the substrate combined with a delta function is proposed and the calculated results match the real-time Si data very well. (C) 2010 Elsevier Ltd. All rights reserved.