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A new evaluation method of forward-biased second breakdown resistance in power transistors employing a characteristic variation of current gain is presented. Predictions for second breakdown resistance in power transistor are made by monitoring the changing point of the base current at the collector-base junction under the constant voltage and constant current conditions. This characteristic phenomenon which is related to the variation of the current gain is dependent on the temperature and current density at a localized spot of the transistor. It is concluded that the time delay required to reach the changing point of the base current can be considered as a measure of the second breakdown resistance. The forward-biased second breakdown resistance can be measured without damage or degradation of the sample transistors, if the supplied power is removed before the base current reaches the changing point.