Basic Properties of Nanostructures

A bandgap can be either direct or indirect. In a direct-bandgap material the maximum energy of the valence band and the minimum of the conduction band occur at the same value of the momentum. Either photons supply the necessary energy for a particle to climb to conduction band from the valence band, or photons are emitted during the transition of the particle from the conduction band to the valence band. No phonons participate in the transitions from the conduction/valence band to the valence/conduction band. Most III-V compound semiconductors, such as GaAs, are direct-bandgap semiconductors and are widely used for optoelectronic applications.