Thermal Stability of Junction Transistors and Its Effect on Maximum Power Dissipation

When the junction temperature of a transistor increases, the collector current increases, as a result of increases in 1) saturation currents and 2) dc conductances. Thermal instability occurs when junction temperature and collector current increase in regenerative and uncontrollable fashion. The limit depends on factors both within and external to the transistor. The internal factors are the thermal resistance, the current amplification factor, and the base lead resistance. The external factors are ambient temperature, collector voltage, circuit resistances, and the thermal coupling between the transistor and temperature compensation elements, if any. Circuits with greater temperature stability permit a higher maximum power dissipation. The factors are discussed which determine the thermal stability and consequently, the maximum dissipation of transistors. The criteria for stability are derived mathematically and some thermal compensation techniques are treated.