DEGRADATION OF GAIN IN BIPOLAR-TRANSISTORS

In this paper hot carrier related aging of n-p-n bipolar transistors i s investigated experimentally and theoretically in order to bring phys ical insight into the bipolar h(FE) (common emitter current gain) degr adation. Electrical stress experiments are performed on transistors wi th different base doping profiles at varying temperatures. Detailed pr ocess simulations are performed to determine the doping profiles of th e base-emitter junction. Monte Carlo transport simulations are then pe rformed at different temperatures and bias conditions to determine the electron and hole distribution functions in the base-emitter junction . AT&T's 0.8mum BICMOS technology is used to fabricate the experimenta l bipolar structures. For this non-self aligned technology we attribut e h(FE) degradation to the presence of hot holes and secondary electro ns which are generated by hot hole impact ionization. This feedback du e to impact ionization has a dominant effect on the high energy tails of the distribution of both holes and electrons even when the overall current multiplication is low. Simple hot electron energy transport mo dels do not contain the complexity to properly describe ionization fee dback and carrier heating, and are therefore inadequate. An exponentia l dependence of the transistor lifetime on BV(EBO) is deduced for cons tant voltage stress (V(stress) < BV(EBO)) conditions, confirming the i mportance of secondaries in the process of degradation.