peed and computational efficiency can be expected to overcome these limitations. In the future, Monte Carlo SEU simulations may become more commonplace, especially as Monte Carlo algorithms are frequently inherently easy to parallelize. II-49 5.2.1 3D Device and Mixed-Level Simulations One of the many challenges of device simulation of radiation effects is the need for advanced, three-dimensional modeling tools. The inherently three-dimensional nature of an ion passing through a microelectronic device is difficult to address with the two-dimensional simulation programs that are routinely used in the semiconductor industry for device analysis. The development of full three-dimensional tools has been fairly recent, however, and much insight has been gained in the past through the use of two-dimensional programs [50,53,54,56,67, 87,89,90,138,154,182,255-263]. The fundamental problem with representing a three-dimensional ion strike in fewer dimensions is shown in Figure 5.2. In a two-dimensional simulation, all quantities are assumed to be extruded into the third dimension, and hence either the correct generated charge density or the correct total charge can be simulated, not both. Scaling schemes have been proposed that adjust the Auger recombination rate in an attempt to correct for geometry effects [258]. Another method is to use quasi-three-dimensional versions of the popular PISCES-II code, based on cylindrical symmetry and coordinate transformations [264]. Many charge collection and SEU studies have been performed using these modified two-dimensional codes [57,59,61,110,115,116,240,241, 265-268]. Unfortunately, there are few devices that exhibit circular symmetry, although through clever use of geometrical approximations, cylindrically symmetric simulations have proven revealing and surprisingly accurate in some cases [110,116]. Full 3D device codes are necessary to model the effects of angled ion strikes [68,241,269] Fully-3D device simulators were first reported in the literature in 1980 [270,271], and some o