second breakdown

A condition of a transistor, resulting from a lateral current instability, in which the electrical characteristics are determined principally by the spreading resistance of a thermally maintained current constriction. The initiation of second breakdown is observed as a decrease in the voltage sustained by the collector.

NOTE Second breakdown differs from thermal failure in that its initiation cannot be predicted from low-voltage thermal-resistance measurements. Unless the current and duration in second breakdown are limited, the high junction temperature at the current constriction will result in failure, usually as a collector-to-emitter short circuit. Second breakdown can occur at positive, negative, or zero base current.

References: 

JESD10, 9/81