r substrate currents. This is, of course, exactly what one has in the case of an energetic particle strike, SO it should not be surprising that latchup can indeed be initiated by a single-event strike. Single-event latchup (SEL) was observed in ground tests as early as 1979 [175], and at first was limited to heavy ions and bulk technologies. As technology parameters evolved, it was found that some devices fabricated on epitaxial substrates could exhibit latchup [176], and worse still, that protons could induce latchup in sensitive technologies [177-179]. It is beyond our purpose here to give a comprehensive discussion of all that is known on SEL. The reader is referred to many excellent reviews and analyses of the subject for further information [172,174-178,180-186] As mentioned above, the latchup condition is triggered when the parasitic BJTs inherent in CMOS turn "on", establishing a positive feedback loop. For SEL, the triggering involves four distinct steps, as described by Johnston, et al. [174]: 1) The particle strike induces a transient current in the well-substrate junction, which in turn results in a voltage drop within the well. The magnitude of the voltage drop depends on the resistivity of the well, well depth, and distance of the strike from the well tie contact. 2) If the voltage drop in the well is large enough, the vertical parasitic BJT becomes forward biased and magnifies current flow into the substrate due to bipolar amplification. 3) Current flowing in the substrate from the vertical BJT produces a voltage drop in the substrate, which in turn forward biases the lateral BJT. Collector current from the lateral BJT provides base current to the vertical BJT, initiating the positive feedback process. 4) Once the regenerative feedback process begins, current increases rapidly and the device can remain latched even after the subsidence of the original particle-induced current. References [174,180,186] give further details of the charge-collection process and initiation of SEL. 4.1.2 Mitigation Techn