We know, that conductors, conduct because their valence energy band is "half" full, and k ("wave vector") can increase and therefore the electrons under the influence of a electric field can "move", and similarly insulators wont conduct, because ther valence band is full, and we have a "big" energy gap between the valence and conductivity band.
My question: Why can't under the infulence of a electric field electrons "jump" onto the next energy band (that's the conductivity band) , where their k ("wave vector") can increase "more freely" and therefore conduct electricity?