# Low energy electrons occupy holes

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 ?

• I'm presuming this is in the context of semiconductors. And, in general, yes once the hole is generated it may move (relax) in the valance band. But then, electrons excited into the conduction band, if originally above the band edge, will relax down to the band edge. Perhaps you could clarify. – Jon Custer Apr 5 at 20:35
• If a hole can move to low level energy levels in the balance.band , I don't understand why the fermi factor is 1 for those levels. Or it isn't? – Seven Apr 6 at 8:45
• Recall that low energy levels for holes are at the top of the valence band. – Jon Custer Apr 6 at 15:29