As to why-Semiconductors' electrons will become 'excited' (yep, thats what its called) as it gains more energy from light or heat, and this frees up more electrons to carry charges across the semi conductor. Each collision uses up some energy from the free electron and is the basic cause of resistance. If the conductor has a negative temperature coefficient the resistance will decrease. This may be expected to happen because, as temperature changes, the dimensions of the conductor will change as it expands or contracts. . Unfortunately there is no simple mathematical function to describe these relationships. In cool weather, the resistances of acid increase and hence the cars not start easily. This behaviour can be better understood if one considers that the interatomic spacing increases when the amplitude of the atomic vibrations increases due to the increased thermal energy. So at the different temperature the resistance of the same conductor will be different. One resistor has a positive temperature coefficient, and the other has a negative temperature coefficient. The general rule is resistivity increases with increasing temperature in conductors and decreases with increasing temperature in insulators. The resistance values are chosen so that when the temperature changes, the increase in resistance experienced by one resistor is offset by the decrease in resistance experienced by the other. The moving charges (electrons) collide with the positive ions in the metallic material. In fact, for the different type of materials, the amount of change in resistance due to change in temperature is different which are discussed as follow. Its value depends on the details of the form of carbon but often is about -0.0005/°C near room temperature. At high temperatures the moving charges will collide more often … These changes in resistance cannot therefore be explained by a change in dimensions due to thermal expansion or contraction. //--> //-->