Current/voltage characteristics of transistors operating in current-mode second breakdown

The current/voltage characteristics of Si n-p-ntransistors, when the device is pulsed into second breakdown for several nanoseconds, are studied. The collector/base characteristics have limits given by Ie= 0 which can be modelled approximately by neglecting recombination, and by Ib= 0 which requires recombination to be fully understood. This later condition is closely analagous to the Kirk effect with a plasma of holes and electrons close to the base region. In all these second-breakdown states the electron multiplication factor is low, typically less than 1.33, so that there is little useful multiplication gain. Second breakdown thus prevents the transistor from being used as a 3-terminal avalanche transistor. Failure at high current levels occurs most readily in the Ie= 0 state. The cutback in voltage and increase in current can be limited by tailoring the collector impurity profile. These two features suggest ways in which transistors may be improved to protect them against short pulse burnout via current-mode second breakdown.