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Matters are classified according to conductivity. Conductivity is an electrical property of matter, which is the ability to allow the passage of electricity. Matters can be an insulator, conductor, or a semiconductor. Semiconductor materials behave both an insulator and a conductor. At absolute temperature, pure semiconductors behave like perfect insulators. Under room temperature or exposure to light energy or with the addition of impurity elements, the resistance of semiconductors can be decreased dramatically, and behave like conductors. Chemical elements and compounds such as germanium, selenium, gallium arsenide, zinc selenide, lead telluride and the most abundant one, silicon are semiconductors. The decrease in resistance with temperature, light, or impurities arises from an increase in the number of current carriers, free electrons and holes. Semiconductors have negative resistance coefficient, meaning, their conductivity increases as temperature increases.