When photons of energy less than the energy gap is made to fall on some crystals the electron from valence band will not go to conduction band exactly, instead they will go to a level below the conduction band level called exciton bands.
Therefore we can say that even though as electrons is released from valence band it does not become free electron it is still bound to the hole (created) in the valence band by the coloumb force of attraction. Thus, this bound electron-hole pair is known as Exciton.
Exciton:
The bound electron-hole pair which occurs due to the combination of electron in exciton energy level and the associated hole in valence band is known as Exciton.
This bound electron hole pair as a whole is neutral and will not contribute to any electrical conduction to the crystal, but will transfer its energy from one to the next ion.
Types:
There are two types of excitons.
- Weakly bound excitons:
Example: (cuprous Oxide) has weakly bound excitons.
2. Tightly bound excitons:
The tightly bound excitons are the excitons in which the electron hole interparticles distance is lesser than the atomic radius .
This type of exciton, is localized on (or) near a single atom, ie the hole will also exist in the same atom as that of an electron, although the pair may be anywhere in the crystal.
Eg:
Alkali halide crystals such as Nacl, KBr etc., have tightly bound excitons.
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