In the first diagram on "http://hyperphysics.phy-astr.gsu.edu/hbase/solids/fermi.html" [Broken], the band gap in a semiconductor is shown. What is the corresponding Fermi momentum? If you plug E_F into the dispersion relation, you get no solution for k_F, right? I ask because an equation is given with a sum of all k such that k < k_F, and there is a band gap. Does one sum over all k?