Why phosphors act as direct energy gap semiconductor?

[gurubhaskar]

about semiconductors

why phosphors act as direct energy gap semiconductor?
why germanium and silicon act as in direct energy gap semiconductors?

 

[drabos]

about semiconductors

germanium and silicon has indirect bandgap structure.
GaAs has direct bandgap structure. Just think that normal laser behaviour requires direct bandgap structure, and many laser diode made from GaAs.
Just see the energy vs impulse diagrams, then you can see that from the largest energy of the the valence band as the smallest energy of the conductance band has the same impulse (in solid state physics they use k instead of the pulse many times) in the case of GaAs and then watch the diagram of the silicon and you will see the difference.