This paper models the indeterminacy that occurs during the formation of breakdown current channels in a bipolar structure, when subjected to an ultrafast high voltage pulse. The experimental results, pertaining to different regimes of the voltage ramp speed, applied across the bipolar structure are modeled. The avalanche injection mechanisms under variable high speed ramps are studied through the formation and propagation of ionizing waves, which lead to either weak or strong injection of mobile carriers, as the high current injection paths get coupled. Furthermore, the role of emitter injection is related to the indeterminacy associated with the snapback phenomenon and systematically related to the experimental observations. © 1963-2012 IEEE.