rily protons and neutrons) generated within packaging or shielding may be an even more significant source of SEU in these sensitive devices [33]. Once a nuclear reaction has occurred, the charge deposition is not greatly different in character from a directly ionizing heavy ion strike. Therefore, once deposited, it is subject to the same fields and concentration gradients and is collected in a similar manner. Charge Collection The basic properties of charge collection following a particle strike have been studied using a variety of experimental and theoretical methods. Broadbeam charge collection spectroscopy measurements have been used to determine SEU-sensitive volumes in SRAMs [34-36], and ion microbeams and lasers have been used with high-speed sampling oscilloscopes to measure charge-collection transients in Si and GaAs devices [37-46]. Ion microbeams and lasers have also been used to map integrated charge collection as a function of position in ICs [45,47,48], and more recently as a function of both time and position [49]. The physics of charge collection have also been intently studied through the use of two- and three-dimensional numerical simulation [50,51]. It is beyond the scope of this short course segment to comprehensively review the massive literature on charge collection; we seek to touch the main highlights and recent developments only. 3.2.1 Basic Physics of Charge Transport There are basically only three mechanisms that act on the charge deposited by an energetic particle strike: 1) carriers can move by drift in response to applied or built-in fields in the device, 2) carriers can move by diffusion under the influence of carrier concentration gradients within the device, or 3) carriers can be annihilated by recombination through direct or non-direct processes. These three mechanisms are of course not unique to the particle strike problem and are in fact the governing processes of charge transport in semiconductors under most operating conditions [52]. When a particle strikes a microelectronic d