Analysis of fast electrothermal dynamics in power BJT

It is well known that the self heating phenomenon in power bipolar devices strongly influences the electrical behavior. Heating phenomenon is a dynamic process ruled by different time constants. While the thermal dynamics associated to external heat exchange is an order of magnitude slower than the electrical one, the thermal dynamics occurring at the silicon level near the active junction is relatively faster and can interact with the electrical one. In this paper, it is proved that such a fast thermal dynamics is responsible for unstable oscillating electrical transients that can be detected in power bipolar transistors. An electrothermal resonance phenomenon Is theoretically and experimentally verified on a commercial power BJT. It is explained how such a phenomenon ran be operatively employed to extract the parameters of the fast thermal dynamics, difficult to measure with conventional procedures.