ction (diffusion) current flows opposite to the initial drift current, returning the SRAM to its initial state. This strike location does not produce SEU. Inside-the-well strikes are particularly interesting because of shunt and bipolar effects that can occur in multilayer structures [67]. For the inside-the-well "off" strike (Figure 3.10c), the initial drift current pulls down the struck node potential, initiating the upset process. As the transient proceeds, holes deposited in the p-well are collected at the p-well ties, raising the well potential and leading to injection of electrons by the source [67,68,70,88]. This initiates the inside-the-well bipolar effect discussed in Section 3.2.2 and illustrated in Fig. 3.6. Electrons collected by the substrate do not contribute to upset because the substrate is attached to VDD However, electrons collected by the n-drain constitute a bipolar current in the same direction as the initial photocurrent, and do contribute to the upset process [70]. For small geometries, the inside-the-well "off" strike can become an important mechanism. In the inside-the-well "on" strike (Figure 3.10d), the ion strike bridges two n-type regions that are at an initial potential difference of VDD. The result is a shunt current that quickly raises the struck node voltage and starts the upset process [89]. This mechanism is self-limiting. As the struck node voltage rises, the potential difference across the shunt disappears. Similar to the inside-the-well "off" strike, holes collected at the p-well ties initiate a bipolar effect, but this time the bipolar current tends to restore the node to its original state. For small geometries, the bipolar effect is strong enough to provide a great deal of intrinsic protection from such strikes [70]. Interestingly, incident particles far below the upset threshold are often sufficiently ionizing to induce a momentary voltage "flip" at the struck node of an SRAM. For example, Figure 3.11 shows drain voltage transients in an SRAM for a particle strike with LE