we know that at temperature above 0K, in semiconductor some electron collecting sufficient energy fron surroundings to cross the band gap, placed in the conduction band. But whats the actual procedure to make the transition. I mean to say is it direct absorption of IR photons or by some other means. If it is direct absorption of IR photons, then how the transition occurs in the semiconductors having band gap energy higher than IR photons. whats the actual procedure.