Figure 2.2-2. Solar Cell Spectral Response Comparisons When photons are incident with energy greater than the bandgap, absorption of the photons can take place and electrons can be raised in energy from the valence band to the conduction band, creating hole-electron pairs. If the excess minority carriers (holes on the n-side and electrons on the p-side of the junction) are able to diffuse to the edges of the space charge region before they recombine, they are swept across the junction, giving rise to a photocurrent, photovoltage, and power into the load. The GaAs cell is a direct bandgap cell and has a steep photon absorption edge. Practically all the carriers generated by sunlight above 1.4 ev are generated in the first 3 pm from the surface, and 50 percent of all carriers are generated within the first 1/2 pm. A technique to improve the spectral response of the GaAs cell is to grow a high-bandgap semi-conductive layer on the surface of the diffused region, and also select a material which closely matches the lattice properties of the cell material such as GaAlAs. A schematic of the basic cell structure is also shown in Figure 2.2-2. The GaAlAs layer is transparent to most sunlight and eliminates the surface states and other imperfections on the GaAs P/N junction surface that would contribute to the cell losses. The p-typd layer also forms an ohmic contact to the p GaAs region.