Direct optical transition

A photon-induced interband transition that approximately preserves the electron's crystal wavevector because the photon momentum is small compared with reciprocal-lattice momenta. Its strength depends on the dipole matrix element and the joint density of states. A direct band gap permits such a transition between the band-edge states, but optical selection rules can still suppress it.

A vertical direct transition at a common wavevector puts an electron above the conduction edge and leaves a hole below the valence edge. Their parabolic kinetic energies are inversely proportional to their effective masses. The excess photon energy is therefore split as $K_e/K_h=m_h^{\ast }/m_e^{\ast }$, rather than equally unless the masses match. The initial valence electron energy is $-K_h$ relative to its band maximum; the final conduction energy is $E_g+K_e$.