We present a detailed understanding of filamentation, through rigorous mixed-mode 3D simulation in a nano-meter scale drainextended NMOS (DE-NMOS). Localization is first triggered in the 2D plane due to regenerative turn-on of the parasitic bipolar. 3D Simulations performed by adding width along the Z-axis (i.e., W) show a very prominent localization effect, which leads to electro-thermal runaway in the DE-NMOS and causes an irreversible damage. © 2008 IEEE.