Thermal properties of high-power transistors

The temperature of a transistor can be determined from the emitter-base voltage versus collector-current characteristic. This characteristic was used for studying the stability of parallel pairs of high-frequency high-power transistors. The thermal effect may cause the incremental emitter-base resistance to assume a negative value. This, in turn, will cause the current flow in a pair of transistors to be asymmetrical. The transition from symmetrical to asymmetrical current flow occurs at a power level which is determined by the nonshared thermal and electrical resistances. Stability to a higher current level can be obtained by increasing the nonshared emitter or base resistances or reducing the collector voltage. Higher currents can also be obtained by reducing the nonshared thermal resistances which indicates close thermal coupling between the two units is desirable.