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Focused ion microbeam and broadbeam heavy-ion experiments on capacitors and SRAMs are used to investigate increased saturation upset cross sections recently observed in some silicon-on-insulator (SOI) integrated circuits (ICs). Experiments performed on capacitors show a very strong bias and oxide thickness dependence for charge collection. In combination with three-dimensional (3-D) simulations, these data suggest that the mechanism for charge collection in capacitors is due to perturbation of the substrate electric fields by charge deposition in the substrate. For substrates biased in depletion, these perturbations induce displacement currents through the oxide. Charge collection by displacement currents can be substantially reduced or mitigated by using heavily doped substrates. Experiments performed on SRAMs also show enhanced charge collection from displacement currents. However, experimental data and 3-D simulations show that for SRAMs, a second mechanism also contributes to charge collection. The 3-D simulations suggest that the charge collection results from drain and body-tie heavy-ion strikes within a few tenths of a micron of the body-to-drain junctions. These charge collection mechanisms can substantially reduce the SEU hardness and soft-error reliability of commercial SOI ICs.