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we have 2 main conservation equations to consider while solving problems which are the energy conservation & momentum conservation.
for transmission of electron from Ec (conduction band) to the Ev (valence band) this conservation (momentum conservation) causes that some materials emit light (photon) (direct materials :GaAs) and some others do not emit light or photon and this energy cause the lattice vibration (phonon).
for more information see https://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
or semiconductor fundamentals books