Photoconductivity is the tendency of a substance to conduct electricity to an extent that depends on the intensity of light-radiant energy (usually infrared transmission or visible light) striking the surface of a sample. Most semiconductor materials have this property. When there is no illumination, a photoconductive sample has a conductance that depends on its dimensions, on the specific material(s) from which it is made, and on the temperature. In most cases, the greater the radiant energy of a specific wavelength that strikes the surface, the higher the conductance of the sample becomes, up to a certain maximum. When the maximum conductance is reached for a particular sample, further increases in irradiation produce no change in the conductance. Photoconductive materials are used in the manufacture of photoelectric devices. Typical photoconductive substances consist of germanium, gallium, selenium, or silicon with impurities, also known as dopants, added. Other common materials include metal oxides and sulfides