 incident particle (such as a heavy ion) is ionizing enough to free a very high density of carriers. For lighter particles (e.g., protons), direct ionization may produce an insufficient amount of charge to cause upset directly and SEU may instead be due to ionization produced by secondary particles. 3.1.1 Direct Ionization As we have said, when an energetic particle passes through a semiconductor material it frees charged carriers along its path as it loses energy. When all of its energy is lost, the particle comes to rest in the semiconductor, having traveled a total path length referred to as the particle's range. We frequently use the terms linear energy transfer (LET) or dE/dx to describe the energy loss per unit path length of a particle as it passes through a material. LET has the seemingly odd units of MeV-cm2/mg, but this is simply because the energy loss per unit path length (in MeV/cm) is normalized by the density of the target material (in mg/cm3. We can easily relate the LET of a particle to its charge deposition per unit path length, because for a given material it takes a certain amount of energy to release an electron-hole pair. For example, in silicon one electron- hole pair is produced for every 3.6 eV of energy lost, and silicon has a density of 2328 mg/cm [17]. Using these values it is easy to show that an LET of 97 MeV-cm2/mg corresponds to a charge deposition of 1 pC/um. This conversion factor of about 100 is handy to keep in mind if you need to go back and forth between energy loss (LET) and charge deposition. A curve of particular interest for understanding the interaction of a given energetic particle with matter is the LET of the particle versus depth as it travels through the target material. Figure 3.1 shows such a curve for a 210-MeV chlorine ion traveling through silicon (a common heavy ion beam used for SEU testing at the Tandem Van de Graaff at Brookhaven National Laboratory). Such curves are readily obtained using computer codes derived from the work of Ziegler, et al (e.g., the TR