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Bipolar ESD protection devices subjected to low current long pulse stress can sustain a relatively long time during thermal second breakdown without any damage. The effect is related to a particular current filamentary behavior, which is observed optically by TIM and explained by device simulation. It is also shown that the second breakdown is initiated at the edges of the device when a moving current-tube arrives at the device edge. Thus circular devices, having no edges, exhibit lower risk of second breakdown.

Traveling multiple current filaments (CFs) are investigated by transient interferometric mapping method in avalanching bipolar n-p-n transistors. The number of CFs can vary for identical current pulses and their averaged number increases with the total current. The CF movement is driven by a temperature gradient in it, caused by the self-heating effect. For pulses of 500 ns duration, the existence of two CFs appears dangerous as it causes a nontrivial premature thermal breakdown (TB), which does not occur when only one CF exists at the same current level. TB occurs due to redistribution of current between the two CFs. The current components flowing through each CF depend on CF temperature and are globally coupled by a fixed device current. When a first CF reaches the device end, it heats up and disappears due to vanishing impact ionization rate in it. When a second traveling CF, taking consequently the whole current, reaches the already preheated device end, a TB event occurs. The transition from two to one filament mode is accompanied by a voltage step of 1 V, which is related to a finite resistance of CF access region.

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