Degradation of gain in bipolar transistors

In this paper hot carrier related aging of n-p-n bipolar transistors is investigated experimentally and theoretically in order to bring physical insight into the bipolar h_FE (common emitter current gain) degradation. Electrical stress experiments are performed on transistors with different base doping profiles at varying temperatures. Detailed process simulations are performed to determine the doping profiles of the base-emitter junction. Monte Carlo transport simulations are then performed at different temperatures and bias conditions to determine the electron and hole distribution functions in the base-emitter junction. AT&T´s 0.8 μm BICMOS technology is used to fabricate the experimental bipolar structures. For this non-self aligned technology we attribute h_FE degradation to the presence of hot holes and secondary electrons which are generated by hot hole impact ionization. This feedback due 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 models do not contain the complexity to properly describe ionization feedback and carrier heating, and are therefore inadequate. An exponential dependence of the transistor lifetime on BV_EBO is deduced for constant voltage stress (V_stress<BV_EBO) conditions, confirming the importance of secondaries in the process of degradation