Dynamics of localized current filaments is analyzed in electrostatic discharge protection devices of a smart power technology during microsecond long constant current pulses. Experiments performed by backside transient interferometric mapping technique and transmission line pulser stressing are correlated with 3D device simulation. Motion of the filament along the device width and its reflection at the device ends are observed. This behavior is related to the time evolution of the voltage waveform and is explained by the simulation. The filament motion is driven by the temperature gradient in the filament due to the negative temperature dependence of the impact ionization rates. A simplified analytical expression for the filament speed as a function of stress conditions and thermal characteristics is given. (C) 2004 Elsevier Ltd. All rights reserved.