HB-based CAD-oriented dynamic stability analysis of circuits and devices: Application to the assessment of thermal instabilities in multifinger HBTs

We present a novel CAD-oriented approach to the analysis of thermal instabilities in power HBTs. The stability analysis is carried out in time-periodic dynamic conditions, by calculating the Floquet multipliers of the limit cycle representing the HBT working point. Such a computation is performed directly in the frequency domain, one the sole basis of the Jacobian of the harmonic balance problem yielding the limit cycle. The corresponding stability assessment is therefore rigorous, and the efficient calculation method makes it readily implementable in CAD tools allowing for circuit and device optimization. Results on 3- and 4-finger layouts are shown, including the assessment of popular stabilization techniques such as emitter ballasting and thermal shunt, thus demonstrating the possible use of the approach as an optimization tool.