A mathematical model is utilized to predict the safe operating area (SOA) for proper circuit applications of bipolar transistors in the forward as well as reverse operating regions. Nonuniformity of the temperature within the transistor structure due to internal self-heating and the avalanche multiplication effect in the reverse operating region, which cause second breakdown failure, are taken into account. Steady-state electrical and time-dependent thermal problems are solved to establish stability of a specified operating condition. Safe operating area curves for three transistor designs of similar power handling capability are presented. Current density and temperature distributions within the transistor structure for various operating conditions in the stable as well as unstable regions are presented. Suitability of VBE to estimate peak temperature within the device is discussed.