Yeah it was, from the success one books :O Are we not supposed to follow those??
So the Success One answers are correct (they are BOSTES, after all), but they aren't always 100% clear. I don't like the way that answer was worded, for example
But here is an explanation: Remember that as Jake said, the
valence and conduction bands are representations/models of reality. The conduction band represents a set of energies that an electron/hole must attain to break free from a covalent bond and travel freely (within reason) through the lattice. The conduction band isn't actually 'above' the valence band; that is just the way we represent it. There is an energy level separating them, not physical space
Essentially, if an electron is moving through the atom, then it is in the conduction band (for our understanding, at least). It must be, because if it wasn't, it would still be forming part of a covalent bond. The acceptor impurity atoms that are introduced generate additional holes in the lattice; an adjacent electron may therefore jump to fill the hole. It had the energy to break its covalent bond to do so, and therefore, in the conduction band
Kinda technical, but does that make sense?