Traps

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 ionisation of donar and acceptor impurities, thereby producing electron and hole pairs. The ionisation 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.

(i) Trapping centre

Here the captured charges carrier may re-excite to free state and again contribute for electrical conductivity (i.e.,) temporary removal of charge carriers.

(ii) Recombination centre

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

It has to be noted that the trap (energy level) can capture either electrons (or) holes easily but not both of them. The captured electrons will be released after some time and moves freely until they are captured by another trap.

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

Step 1: Initially an electron from the conduction band is captured by the empty trap of energy level E_t as shown in Fig. 4.1.

Step 2: The captured electron recombines with the hole which appears nearby to the trap as shown in Fig. 4.2.

Step 3: The recombination of electron and hole occurs via trap and a little amount of energy is emitted and the trap becomes empty again as shown in Fig. 4.3.

Since the trap centre is tightly bounded to the lattice points and the energy gap is not exactly equal to E_g the energy will be released in the form of heat, which heats up the material by non-radiative transition. 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.

Note: It is not so that electron should come to the trap and then recombines with the hole nearby the trap, rather, the hole can also come to the trap first and then it can recombine with the electron (in CB) nearby the trap.