OPTICAL MATERIALS:- TRAPS



Trap is an intermediate energy level present in the energy gap between the valence band and conduction band, which arises due to the presence of impurity atoms (or) imperfections in the crystal.

    Let us consider a crystal with donar and acceptor impurity atoms. If the light is made to incident on the crystal, it may cause ionization of donar and acceptor impurities, thereby producing electron and hole pairs. The ionization depends on the nature of the impurity in the crystal.

    The electrons and holes thus created will move freely, until they are captured at an imperfection. There are two types of capturing centres.
  •     Trapping centre:
        Here the captured charge carrier may re-excite to free state and again contribute for electrical conductivity (ie) temporary removal of charge carriers.

  •  Recombination centre:

        In this case, the captured charge carriers will recombine with the opposite charge carrier in the recombination centre itself (ie) permanent removal of charge carriers.

Types:

    There are two types of traps.
:
Type I

    This kind of traps will capture the electrons from conduction band and subsequently transfer the electrons to the valence band. Thus the recombination of electrons and the hole which appear near the valence bands takes place via traps.

Explanation:

Step1:

    Initially an electron from the conduction bands is captured by the empty trap of energy level .

Step2:

    The captured electrons recombine with the hole which appears near by to the trap.

Step3:

    The recombination of electron and hole occurs via trap and a little amount of energy is emitted and the trap becomes empty again.




Since the trap centre is tightly bounded to the lattice points and the energy will be released in the form of heat, which heats up the material by non-radiative transmission. Therefore this type of traps (defect centres) helps in restoring the thermal equilibrium.

Type II:

    This kind of traps will not produce heat transition but they restrict the freedom of motion of electrons (or) holes. Since the electron motion is restricted the conductivity of the material is reduced.


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