Modeling Neutron Radiation Damage in Bipolar Junction Transistors Isolated from an Integrated Circuit

Neutron radiation, common in space-environments and reactors, can have a profound impact on the performance of integrated circuits (ICs) containing silicon bipolar transistors (BJTs). To model the current-voltage (I-V) characteristics in these environments, it is crucial to study neutron-induced defect distribution and its effects on carrier transport. Several studies [1] [2] model post-radiation behavior of BJTs by adjusting carrier-lifetimes within the base of the BJT structure. Lifetime fitting studies provide only a basic understanding of the gain degradation of BJTs in the presence of neutron radiation, but cannot capture changes in the ideality-factor should they be observed in gain degradation measurements on irradiated BJTs. Furthermore, the existing studies were performed on discrete BJTs, which are generally different from BJTs integrated in a circuit (IC). To address these research-gaps, we isolated lateral pnp and vertical npn BJTs in several LM741 Op-amp ICs and performed a 14 MeV neutron-irradiation study on those structures. Gummel-sweeps revealed an increase in base current, an expected result, which also included a change in ideality-factor as a function of fluence. To understand the underlying physics, TCAD simulations were used to fit the measured I-V characteristics before and after neutron irradiation. Our simulation results show that the ideality-factor relies heavily on the fluence dependent carrier lifetime, especially in low voltage regime. This carrier-lifetime depends on the spatial and energy distributions of those traps. Different trap-distributions were used in both the depletion and neutral region of the base to model the experimentally observed change in ideality-factor with increasing neutron fluence.