T well below the upset II-20 threshold, just below the upset threshold, and just above the upset threshold. Even the particle with LET well below the upset threshold causes a significant voltage transient at the struck drain. Whether an observable SEU occurs depends on which happens faster: the feedback of the voltage transient through the opposite inverter, or recovery of the struck node voltage as the single-event current dies out [63,89,90]. Note that in terms of the fundamental charge-collection characteristics discussed in Section 3.2, drift (including funneling effects) is responsible for the rapid initial flip of the cell, while long-term charge collection by diffusion prolongs the recovery process; both mechanisms are critical to the upset process. The recovery time of an SRAM cell to a particle strike depends on many factors, such as the particle LET, the strike location, etc. From a technology standpoint, the recovery time depends on the restoring transistor current drive and minority carrier lifetimes in the substrate [89,90]. A higher restoring current leads to a faster recovery time, as do decreased minority carrier lifetimes. a) Outside-the-well OFF Strike b) Outside-the-well ON Strike n-epi n-epi 0-VDD ip = drift photocurrent id = diffusion current ib = bipolar current c) Inside-the-well OFF Strike d) Inside-the-well ON Strike is = shunt current VDD->0 -VDD-0 p-well p-well n-epi n-epi Figure 3.10. Illustration of possible SRAM upset strike locations and mechanisms [70]. II-21 Threshold LET = 42 MeV-cm2 2 /mg LET = 9 MeV-cm 2 /mg LET = 40.5 MeV-cm2/mg LET = 45 MeV-cm2/mg 10-14 10-13 10-12 10-10 Time (s) Figure 3.11. SRAM struck drain voltage transients for ion strikes with LET well below, just below, and just above the SEU threshold. Even the ion strike with LET well below the actual SEU threshold is sufficiently ionizing to momentarily flip the struck node voltage. This is because a higher restoring current is more quickly able to re-establish the struck node voltage, while decreased substrate mino