Transient junction temperature rise and second breakdown in transistors

Second breakdown is a most serious problem in transistor failure, especially in switching circuits with inductive loads. In this paper, second breakdown is analysed by a thermal concept. The transient junction temperature rise is determined by the transient thermal resistance measured by a new method suggested in this paper. Then, the second breakdown in base open condition is analyzed by increase of the collector saturation current ICEOdue to junction temperature rise. The delay time of second breakdown, which is defined as the time for collector current to increase to 120 percent of the initial value, is calculated. An experiment to measure the delay time is made exactly according to the calculations, and it is shown that the experimental result agrees with the calculated result. Furthermore, second breakdown in the base reverse bias condition and the turnover mechanism of collector output characteristics in a switching circuit with an inductive load are explained by current crowding effect. Second breakdown in a silicon transistor is also supposed to be caused by the same thermal mechanism.