f the early work on three-dimensional device simulation was motivated by alpha-particle reliability issues [272-274]. An early comparison of 2D and 3D charge-collection simulations showed that while the transient responses were qualitatively similar, significant quantitative differences existed, both in the magnitude of the current response and the time scale over which lon Strike Two-Dimensional Quasi-Three-Dimensional Three-Dimensional Correct charge density Correct charge density Correct charge density Correct total charge Correct total charge Correct total charge Incorrect geometry Correct geometry Incorrect geometry Computationally efficient Computationally intensive Computationally efficient Figure 5.2. Illustration of the inherently three-dimensional nature of ion strikes [51]. II-50 collection was observed [275]. The implication of these results is that while 2D simulations may provide basic insight, 3D simulation is necessary if truly predictive results are to be obtained. Throughout the 1980s companies developed internal 3D device simulators [276-280], but most of these were proprietary and optimized for super-computers. Only in the present decade have numerical techniques and microprocessor speeds been sufficiently improved to bring such tools to the desktop workstation. In the last few years fully three-dimensional device simulators have become commercially available [245,281-283]. Optimized for high-end workstations, a fairly large 3D simulation can generally be performed in about a day. Another consideration that arises in modeling charge collection and SEU is the tight coupling of device and circuit response to incident ionizing radiation. In SRAMs, for example, modeling the struck transistor with typical constant boundary conditions (or even including passive, lumped elements) will never result in an upset being observed in the simulation - by construction the device will always return to its pre-strike state. The best that can be done in this situation is to study the charge-collection characteri