Three hFE Degradation Mechanisms and their Associated Characteristics

Degradation of hFE in bipolar transistors has been a recurrent problem with all makers and users of solid state devices. To date, three distinct mechanisms have been identified with hFE degradatiofi and are the subject of this paper. The first and most important hFE degradation mechanism, that associated with positive ion migration within the passivating device oxide, requires total protection of that oxide if device reliability is to be assured. The second mechanism, that associated with emitter-base reverse bias avalanche, is due to the introduction of fast surface states. While these two mechanisms affect hFE primarily at low collector current levels, a third mechanism has been identified which affects hFE at moderate current levels as well.