Optimum design for a thermally stable multifinger power transistor

The thermal stability of multifinger bipolar transistors has been analyzed theoretically. Coupled equations are solved to study the onset of instability and its dependence on the distributions of ballasting resistors. Analytical expressions were derived for the emitter ballasting distribution for optimum stable operation. Compared to conventional methods with uniform ballasting, the optimized design can significantly increase the stable operating current of the transistor. An absolutely stable operating condition is also derived. At this condition, the device never becomes unstable.