 rather than the three of the drift- diffusion method. Energy balance options are available in commercial drift-diffusion-based codes [245], but have not been extensively used for SEU calculations. 2D energy transport codes have been used for ion-induced charge-collection simulations in GaAs devices, where off- equilibrium effects are particularly important [138,246,247]. The top rung of the device simulation ladder is Monte Carlo simulation, which makes the fewest assumptions and approximations [248]. Rather than being based on approximations to complicated macroscopic equations, Monte Carlo methods describe carrier transport on a fundamental, microscopic scale using classical equations of motion (e.g., Newton's first law). The motion of individual carriers is followed as they drift in fields and interact with scattering centers until statistical significance is achieved. Few assumptions are involved other than transport is described using classical physics. The penalty is very high computational intensity as the trajectories of many thousands of particles must be tracked to attain meaningful statistics. The utility of the Monte Carlo method for simulating radiation-induced charge collection was realized very quickly [249]. Early simulations computed two-dimensional "collection maps" of alpha-particle generated carriers in regular arrays of cells, while a follow-up study looked at scaling effects on SEU in DRAMs [249,250]. The methodology was also extended to cosmic-ray induced SEUs [251]. To reduce computation time, these simulations treated the third dimension (depth into the substrate) with analytic models. Monte Carlo simulation has also been applied to studying SEU in short-channel CMOS on SOI technologies [252]. Recently, a fully three- dimensional Monte Carlo simulator has been described [253], and used in combination with a circuit simulator [254]. Due to high computational requirements, the response during only the first hundred or SO picoseconds was calculated, but continuing improvements in computer s