In Tipler's Physics it is said that for a temperature $T>0$ the only electrons that can gain energy from collisions are the ones with initial energy greater than $E_F-K_BT$. I understand that, because of the Exclusion Principle, electrons with less energy cannot get excited at the beginning, because there are no empty levels for them to occupy. However, once an electron with energy greater than $E_F-K_BT$ has increased its energy and left a hole at its original level, couldn't another electron with less energy gain energy enough( by collisions) to occupy that hole? If the answer was that it can, then the Fermi factor wouldn't be $1$ in any case, so I guess it can't do it. But I don't understand why. EDIT: I'm not sure, is this only valid for semiconductors ? Couldn't it be applied to metals, for example ?