The dependence of the increase of current in a transistor of n+-p-n-n+ type on time is investigated, and current mode second breakdown (CSB) and thermal mode second breakdown (TSB) are discriminated by the duration of the delay time before which second breakdown occurs. The boundary conditions between CSB and TSB in steady state are clarified. The critical voltage between CSB and TSB is determined by the width of the n layer, the immobile charge density of the n layer, and the injected electron density from the emitter to the n layer. The numerical analysis was in qualitative agreement with the experimental results.
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