Thermal Parameter Extraction for Bipolar Circuit Modelling

Introduction: Thermal feedback from collector power to emitter temperature has been shown to significantly affect BJT performance.’-4 Recently several worker^^.^ have demonstrated ways to model this thermal feedback in circuit simulation assuming that thermal model parameters are available. This Letter introduces a simple way to extract thermal parameters needed for such simulations. BJT thermal behaviour is dominated by two components, characterised by the thermal spreading impedance and the chip/package thermal impedance. It is important to extract these components separately, as the package thermal impedance causes a coupling between transistors on the same chip, and thermal spreading impedance usually does not. Standard methods for extracting impedance are adapted for characterising power transistors.’ They are poorly suited for measuring thermal spreading impedance, which becomes increasingly important with decreasing transistor size.