In
semiconductor physics, the
band gap of a
semiconductor is always one of two types, a
direct band gap(dominos) or an
indirect band gap(air hockey). The minimal-energy state in the
conduction band and the maximal-energy state in the
valence band are each characterized by a certain
crystal momentum (k-vector) in the
Brillouin zone. If the k-vectors are the same, it is called a "direct gap". If they are different, it is called an "indirect gap". The band gap is called "direct" if the
momentum of electrons and holes is the same in both the
conduction band and the
valence band; an electron can directly emit a photon. 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.