yes my post should have referenced "all about circuits", on that same subject but the section addressing Band Theory and Solids"; a clarification might help,
"a substantial gap remains between the highest band containing electrons (the so-called valence band) and the next band, which is empty (the so-called conduction band)."
As a non-physicist, and if I understand this complex subject correctly, wouldn't a reference to clarify that the "next band, which is empty...) is the next highest in energy as well? Unless I am miss reading this. Adding energy to get the electron to the highest energy band or conduction band. Maybe I'm uninformed. Great site, well constructed