When heat is applied to a semiconductor material, the outermost electrons in the atom gain energy. These electrons are able to overcome the attraction of the nucleus and leave the atom. Thus they become free electrons which can conduct.
The number of electrons increase exponentially and this results in a large drop in the resistance.
Electronic devices will behave erratically above a certain temperatures. Hence, all electronic devices such as laptops will have a safe temperature beyond which they cannot function.
The effect of vibration of the atomic lattice on the mobility of the electrons is offset by the large numbers of electrons which enter the conduction band.