Origins of total-dose response variability in linear bipolar microcircuits

LM111 voltage comparators exhibit a wide range of total-dose-induced degrad ation. Simulations show this variability may be a natural consequence of th e low base doping of the substrate PNP (SPNP) input transistors. Low base d oping increases the SPNPs collector to base breakdown voltage, current gain , and sensitivity to small fluctuations in the radiation-induced oxide defe ct densities. The build-up of oxide trapped charge (NOT) and interface trap s (NIT) is shown to be a function of pre-irradiation bakes. Experimental da ta indicate that, despite its structural similarities to the LM111, irradia ted input transistors of the LM124 operational amplifier do not exhibit the same sensitivity to variations in pre-irradiation thermal cycles. Further disparities in LM111 and LM124 responses may result from a difference in th e oxide defect build-up in the two part types. Variations in processing, pa ckaging, and circuit effects are suggested as potential explanations.