The charge states of defects can be understood diagrammatically by treating E F as a variable, plotting the formation energies of a defect in different charge states, and identifying the charge state with the lowest formation energy at each E F . As shown schematically in Figure , the formation energy of V +2 B is lower than that of V 0 B when E F is close to the valence band maximum (note that the slope of the formation energy corresponds to the charge state by Equation ), indicating that V +2