This chapter will derive the design equations and identify critical performance parameters for solar cells using silicon and gallium arsenide materials. Silicon material is selected for fabrication of solar cells because of maturity of silicon and lower fabrication costs. Silicon comes in various forms, such as poly-silicon, singlecrystal silicon, amorphous silicon, crystalline silicon, super-crystalline silicon, and ribbon-silicon. Crystalline silicon can be made either from monocrystalline silicon or multicrystalline silicon wafers. It is important to mention that more than 94 percent of solar cells are made from crystalline silicon semiconductor. As mentioned in the first chapter, the maximum theoretical conversion efficiency is 16 percent for silicon-based solar cells and 28 percent for gallium arsenide-based solar cells. Currently these two materials dominate the photovoltaic cell market, for the reasons cited above. Regardless of the materials used, the spectral response from these solar cells depends on the depth of the p-n junction, the absorption coefficient of the fabrication materials, cell junction area exposed to sunlight, and wavelength
of the solar incident light. A solar cell is a semiconductor device that uses a p-n junction to convert solar energy directly into electricity. The magnitudes of the open-circuit voltage and short-circuit current are strictly dependent on the absorption capability of the materials used in the fabrication of the solar cells. The active surface of the cells currently in use consists of a thin layer of p-type silicon on top of a body of n-type material.
Solar cells are made with junction depths varying from 0.6 to 5.0 pm and can have smooth or rough surfaces. Existing response curves  indicate that in order to increase the short wavelength response at a wavelength less than 0.75 pm the junction should be made closer to the surface, while in order to increase the long wavelength response at wavelength greater than 0.75 pm should be made far below the surface. The effect of rough surface is to reduce the life time near the surface, thereby reducing the response to short wavelengths of the sunlight. A theoretical model of the cell shown in Figure 2.1 must be used to describe the mechanism involved in determining the shape of the response curves.