Might be an easy question (or not). What determines the band gap of an element? As far as I know, silicon is the most efficient single-element semiconductor because of it's small (but nonzero) bandgap. Next (as far as I know) is Selenium. I'm aware there are more efficient compounds, but that's outside the scope of the question anyway. So what about the element is responsible for band gap size? What could you change (theoretically) if creating a new element (I said theoretically!) that would reduce band gap size? More electrons per first valence band? Or per conduction band? Is this restricted by other forces?