(color online) Energy band diagrams for an exciton bound to an isoelectronic impurity N in GaP, where E_N is the energy level of the bare electron bound state measured from the VBM, E_h the hole binding energy to the N⁻ center, and E_N,ex = E_N − E_h the lowest bound exciton transition energy. (a) In one electron picture, and (b) in many-electron picture.