Second Breakdown in Silicon Power Transistors at High Collector Voltage

Noticeable difference in pulsed second breakdown performance have appeared in high-frequency power transistors operated in the vicinity of BVCEO. Devices with thin, lightly doped epitaxial collector regions show second breakdown at much lower currents than many devices having the same base and emitter structure but thick epitaxial collector regions. However, some of the latter devices also have low second breakdown currents. The early second breakdown can be predicted in terms of the negative resistance seen at the collector terminal with the base open. This negative resistance leads to internal instability and current crowding. At high voltages, the collector negative resistance is electrical in nature and does not depend on heating effects with consequent time lags (although these will introduce a further negative resistance), so that the instability it introduces builds up very rapidly. It is caused by local reverse base current producing increased forward bias on the emitter junction as it flows through the sheet resistance of the base layer. Two effects modify the collector negative resistance, giving rise to considerable differences in second breakdown performance. One of these effects is the avalanche breakdown occurring at the edge of the shallow diffused planar collector-base junction. If this breakdown occurs at a lower voltage than that required to make M > 1/α, where M is the avalanche multiplication factor for the collector junction under the emitter, no negative resistance appears and the transistor shows a high second breakdown current.