(color online) Energy band diagrams for an acceptor. (a) Conventional hydrogen model for an acceptor. When an electron is excited into the hole bound state at E_A, a free hole is generated in the valence band. (b) Bound exciton model for an acceptor. When an electron is excited into the impurity state at E_I, a bound exciton is formed with a hole binding energy E_A. (c) The bound exciton model in a many-electron picture.