Tetrahedral semiconductors: Constancy of the midgap energies with respect to the vacuum level

It is shown that the midgap levels $E_B$ used to line up band structures at semiconductor heterostructures maintain a constant energy with respect to the experimental vacuum level. This implies that the electron affinity rule for the determination of band offsets is closely related to recent band lineup theories by Flores and Tejedor, Tersoff, and Cardona and Christensen.