If the temperature is increased, the metal ions vibrate more. There are more collisions between the electrons and positive ion and it is harder for current to flow.In a conductor, the resistance increases with temperature as the atoms vibrate more.In a semi-conductor there are fewer charge carriers. When the atoms are supplied with thermal energy, electrons which are bound to the atoms (bound electrons) break free and become conduction electrons. This increases the number of electrons (number density) so the resistance decreases with temperature.Thermistors and light dependent resistors are made from semi-conductor material. If the temperature is increased, the resistance decreases so it is called an NTC Thermistor (negative temperature coefficient).