Transistor design and thermal stability

A model has been developed for the computation of forward second breakdown due to lateral thermal instability in power transistors. The method of analysis is to derive the steady-state current density and temperature distribution of a given transistor design under specified operating conditions, and then to calculate the response of the device to a temperature impulse suddenly applied internally. The current flow calculations have been carried out by using a distributed transistor model, and for the time-dependent heat flow problem the finite difference approach was used. The effect of device design parameters such as chip thickness, base width, emitter width, base impurity concentration, etc., on the thermal stability has been calculated. Also, the effect on transistor stability of the current and voltage operating point, as well as the heat sink temperature, has been analyzed. Information on the stability of a power transistor under pulsed condition is derived by calculating the time constant for thermal runaway. The results of this analysis indicate that the delay time is of the order of 1 ms.