A new dynamic, self-consistent electro-thermal model of power HBTs and a novel interpretation of thermal collapse loci in multi-finger devices

A new self-consistent dynamic electro-thermal model for power HBTs is presented coupling a circuit-oriented electrical model fitted on experimental data with a full frequency domain thermal model. The thermal model provides the exact frequency behaviour of the device thermal impedance through a quasi-3D approach, rather than a simple single- or multi-pole rational approximation. The self-consistent solution is achieved, in large-signal periodic conditions, through harmonic balance analysis. The dynamic model enables us to discuss the behaviour of thermal collapse in multi-finger devices not only in DC, but also in AC conditions. In particular, a new interpretation is proposed for the thermal collapse loci of such devices in terms of bifurcations, thus providing a simpler and more straightforward stability criterion with respect to those already presented in the literature.