Second breakdown phenomena in transistors

This paper presents an extension of the thermal runaway theory for second breakdown phenomena in transistors. This extension is based on the effects of free-carrier density and distribution, on the field in the collector space-charge region. The extension is particularly useful in explaining second breakdown phenomena for high speed switching applications (where second breakdown has been observed to occur in less than 100 nanoseconds). Development involves the solution of the Poisson equation (considering free carriers) for the collector space-charge region. A linear distribution of fixed charge is assumed. Three linear distributions of free carriers (in the collector space-charge region) are considered. They are positive slope, zero slope and negative slope. These three distributions may be used either separately or through superposition to approximate actual free carrier distributions. All three distributions are shown to lead to a negative resistance region as free carrier density is significantly increased (free cartier densities greater than a few per cent of the fixed charge density at the space-charge boundary are considered significant).