A simple model for second breakdown

The analysis of second breakdown in transistors is very difficult when one takes into account the distributed nature of the transistor and the complex interactions of the charge carriers with the crystal lattice and with the various electric fields within the transistor. A simple model has recently been developed which can be analyzed in detail in terms of lumped transistor parameters. The model treats the transistor as though it consisted of two separate, but interacting, parts coupled thermally and electrically. All anamolous portions of the transistor are lumped into one of these. The relative importance of each of the initiating mechanisms which have been hypothesized for second breakdown can now be determined. This paper presents a brief static analysis of the model. Calculated static current-voltage characteristics are shown for some of the initiating mechanisms. The effect of the current dependence of current gain, the effect of the temperature dependence of the collector saturation current, and the effects of avalanche multiplication and its dependence on free charge carrier density will be discussed. It is concluded that all of these effects are important, but one or the other will tend to dominate depending on the transistor structure, type of defects present, and mode of operation. Experimental evidence of a change in the dominant mechanism during swept operation in second breakdown will be shown.