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In any normal resistor, which is made from a conductive material, the resistance increases with the temperature, because the "friction" between the electricity carriers and the atomic lattice increases. Only in resistors made of semiconductors, i.e. in NTC thermistors, the resistance decreases with the temperature. The difference between a conductor and a semiconductor is that in the former the concentration of the carriers is constant, while in the latter the concentration of the carriers can be changed by various means, e.g. by temperature, by light, by impurities, by corpuscular radiation, by electrical fields, by contact with other materials, and by other means. In most intrinsic semiconductors, the carrier concentration increases with the temperature, therefore the resistance decreases with the temperature. This is a valid method to determine that a material is a semiconductor, even if false negatives are possible, when a material is a semiconductor though its resistance increases with the temperature. |