Thermal collisions continue to generate some number of electron-hole pairs due to the jumping of electrons from valence band to conduction band while other electron-hole pairs disappear due to the recombination process. Recombination is the process where an electron moves from the conduction band to the valence band so that a mobile electron-hole pair disappears. The electron in the conduction band will lose its energy when it collides with atoms and hence it comes back to the valence band filling an empty place, thus recombination takes place. Therefore, recombination is the main reason for the conduction electrons not go on increasing with time as thermal collisions continuously generates electron-hole pair.