Turn-off processes in high-voltage n-p-?-n switches

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Abstract
A one-dimensional solution is found for the current-continuity equations that govern minority-carrier removal from the collector region of a saturated bipolar transistor. The shape and position of the charge body is established so that the collector-voltage rise can be predicted, especially in the final stages of turn-off. Constant collector current, corresponding to an inductive load, is assumed during the charge-removal process. The effect of varying the reverse base drive is considered as is turning off from hard- and quasi-saturation. It is shown that the collector region can be swept clear of minority carriers at a collector voltage well below BVCEO. Once minority carriers have been cleared, collector current is supported by a displacement current which causes energy storage, rather than dissipation. Experimental verification is sought with a developmental transistor that minimizes two- and three-dimensional effects. Qualitative agreement is evident; reasonable quantitative agreement does not require unrealistic assumptions. Lastly, the conditions that can precipitate reverse-biased secondary breakdown are identified in an appendix.

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