very specialized large area diodes, some of which have complex multi-junction designs to optimize their conversion efficiency. Light strikes the solar cell and creates electron-hole pairs that generate electrical power only if collected at the cell electrodes. In order to have high collection efficiency, especially at the red end of the spectrum, long diffusion lengths are required in the lightly doped portion of the cells intended for light collection. As expected for a minority carrier device, the degradation in power output is a result of a radiation induced reduction in the minority carrier lifetime. Once again, device radiation sensitivity is greater for materials with longer initial lifetimes, as seen in figure 27. For example, minority carrier lifetimes in GaAs are typically much shorter (tens of nanoseconds) as compared to the much longer Si lifetimes of tens to even hundreds of microseconds. Key electrical parameters include not only the power output, but also the open circuit voltage, and short circuit current. The interested reader may find a wealth of information in the JPL solar cell handbooks [Tada82, Ansp89, Ansp96], and the Photovoltaic Specialists Conference proceedings. Crystalline Si and GaAs/Ge solar cells are most commonly flown today, but multi-junction GaAs cells, InP and amorphous Si cells are being investigated for future use in space. The drive to fly spacecraft in ever more harsh environments (including the more intense part of the proton belts) has spurred interest in more radiation hardened cells. The presence of multiple junctions provides additional design flexibility to achieve increased hardness, as described in [Marv99]. Figure 27 compares the maximum power output degradation as a function of displacement damage dose for several of the most common solar cell materials. The figure summarizes data from several sources [Srou98, Wojt96, Mess97, Hoff97, Nogu90], and clearly shows the nonlinear degradation. Note that InP has the potential for increased survivability as a result of inje