ANALYSIS OF THERMAL-INSTABILITY IN MULTI-FINGER POWER ALGAAS GAAS HBTS/

A new theory is developed in this paper to explain the collapse of cur rent gain in multi-finger power AlGaAs/GaAs Heterojunction Bipolar Tra nsistors (HBT's). The reasons behind this unwanted phenomenon are full y clarified using a simple model to investigate the thermo-electrical interaction between the fingers. The existence of multi-value equilibr ium points in model's constitutive equations is shown to be the necess ary condition for the collapse of current gain to appear. For a N-fing er device, N different patterns of collapse exist, The criterion to se lect the global stable pattern is given. The method has been used to p redict the collapse in AlGaAs/GaAs HBT's and the agreement is excellen t. The method also predicts that the collapse can happen far earlier t han is normally expected in multi-finger high-power devices. The influ ence of ballasting resistance and thermal resistance is also investiga ted.