hensive discussions of funneling, but here are a few key points to keep in mind: 1) The term "funnel length" is a misnomer (as is funneling itself to some extent). There is no well-defined length that can be associated with funneling from a physical standpoint [58,61,62]. Funnel length is thus only a useful concept inasmuch as it can be used as a quasi-physical "fudge factor" in error rate calculations. In this case it can be used to represent any charge collection from outside the defined sensitive volume, including the effects of funneling. Funnel Depletion Region Drift Diffusion Recombination Figure 3.2. Illustration of an ion strike on a p/n junction showing drift, diffusion, recombination, and funneling. II-13 2) Expecting funnel-assisted drift charge collection to be "prompt" can be misleading. In cases where the substrate is lightly-doped, funneling can take a long time (nanoseconds) to develop and to collect charge [54]60]. 3) Funneling does not require a direct strike on a depletion region. Near misses can also cause funneling if a high enough carrier density diffuses into the depletion region to collapse it [54,61]. 4) Funneling in epitaxial diodes is limited by the heavily-doped substrate underneath the epi layer. Once sufficient minority carriers are removed from the epi layer, the depletion region is reformed, funneling stops, and charge collection continues at a much slower rate. This leads to a characteristic knee in the charge-collection characteristics of n/p epitaxial diodes, as shown by the dashed curve in Fig. 3.4 [60,61]. 5) Because of differences in the hole and electron mobility, funneling occurs in reverse- biased n/p diodes, but is much weaker or nonexistent in equivalent p/n diodes. This is indicated in Fig 3.4 by the lack of a knee in the p/n charge-collection characteristics (solid curve). [59,61]. 6) The total charge collected in an epitaxial diode can be approximated by the charge liberated in the epitaxial layer plus the charge liberated within a diffusion length of the epi/substrat