In an "indirect" gap, a photon cannot be emitted because the electron must pass through an intermediate state and transfer momentum to the crystal lattice. Examples of direct bandgap materials include hydrogenated amorphous silicon and some III–V materials such as InAs and GaAs.OverviewIn, the of a can be of two basic types, a direct band gap or an indirect band gap. The minimal-energy state in the and the maximal-energy state in the are. Interactions among,,,, and other particles are required to satisfy and (i.e., conservation of total k-vector). A photon with an energy near a sem.