Optimum design for a thermally stable multifinger power transistor with temperature-dependent thermal conductivity

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. We extended our previous work on the multiple-finger transistor thermal stability from the simple coupled thermal-electrical feedback equation to the more accurate I-V equation and taking the temperature dependence of the thermal conductivity into consideration. Transistors with three-fingers and N-fingers have been analyzed. Two design procedures, uniform current design and uniform temperature design, of the best ballasting resistor distribution for optimum thermal stability operation were developed. Using these design flows, we can design the best ballasting resistor needed for thermal stable operation under the specified current level or specified junction temperature.