A critical discussion of the current collapse in multifinger HBTs based on Floquet stability analysis

Abstract

The paper presents a critical discussion of the electro-thermal instability in multifinger HBTs based on Floquet Multipliers analysis. We show that the usual interpretation of current collapse as a bifurcation phenomenon for layouts with more than two fingers strictly holds only if inter-finger thermal coupling is neglected. Thus, predictive criteria based on the identification of singularity points associated to the nonlinear system may result inaccurate, or even fail, for the analysis of practical devices. On the other hand, numerical results show that a predictive analysis associated to the behavior of the FMs may still be provided. Finally, the proposed approach is applied to the stability analysis of different designs exploiting emitter ballasting or thermal shunt stabilization.