|If the Fermi level is the highest energy level used by an electron, how can electrons shift to conduction levels that are at energies above the Fermi level? — PH
The Fermi level is the highest energy level occupied when all the electrons have as little energy as possible. That situation occurs only when all the electrons are paired two to a level and the levels are filled all the way from the lowest energy level up to the Fermi level. At any reasonable temperature and in the presence of light or other energy sources, some of the electrons will have been shifted out of their normal levels and into levels above the Fermi level. The Fermi level doesn't change when these shifts occur—it's defined before the electrons shift.