evice, the most sensitive regions are reverse- biased p/n junctions, as illustrated in Figure 3.2. The high field present in a reverse-biased junction depletion region can very efficiently collect the particle-induced charge through drift processes, leading to a transient current at the junction contact. The situation is not too dissimilar II-12 from the operation of a solar cell, where large-area p/n junctions are reverse-biased to collect charge liberated in the semiconductor by sunlight. Strikes near a depletion region can also result in a significant transient current as carriers diffuse into the vicinity of the depletion region field where they can be efficiently collected. Note that even for direct strikes, diffusion plays a role as carriers generated beyond the depletion region can diffuse back toward the junction. Shortly following the discovery of SEU, researchers at IBM used numerical device simulators to compute the response of reverse-biased p/n junctions to alpha-particle strikes [50,53,54]. An important insight gained from these early charge-collection simulations was the existence of a transient disturbance in the junction electrostatic potential, which was termed the "field funnel," as shown in Figure 3.3 [53]. Charge generated along the particle track can locally collapse the junction electric field due to the highly conductive nature of the charge track and separation of charge by the depletion region field. This funneling effect can increase charge collection at the struck node by extending the junction electric field away from the junction and deep into the substrate, such that charge deposited some distance from the junction can be collected through the efficient drift process. The funnel effect has been investigated in further detail by later researchers [55-59], including the influence of epitaxial substrates on the transient charge-collection characteristics [60-62]. Several important additional insights have been gained from these studies. The reader is referred to [59-62] for more compre