Carrier Generation and Recombination Processess

CARRIER GENERATION AND RECOMBINATION PROCESSESS

Carrier Generation

We know the valence electrons for the semiconductors are four. Below the absolute temperature all the four valence electrons are tightly bounded with the central positive nucleus, hence it act as an insulator.

But above absolute temperature, some covalent bonds in the structure (semiconductor) breaks and setting up free electrons and holes. Here the number of free electrons produced is equal to the number holes created. In this way charge carriers are generated in pure semiconductors.

Note: In an type semiconductor, the concentration of minority carriers (holes) are increased due to thermal agitation. Similarly, in a p-type semiconductor, electrons (as minority carriers) are increased due to thermal agitation. These minority carriers play vital role in the working of light Emitting Diode.

Recombination processes

Recombination is simply a process of converting free electrons into valence electrons. Consider an energy band structure for the semiconductors. It is observed that the energy of the conduction band (where the free electrons are situated) is high when compared valence band (where the valence electrons and holes are situated).

Suppose an electron moves from the conduction band to the valance band, an empty place (i.e., hole) in the valence band is occupied. Here, since an electron travels from higher energy state to lower energy state some amount of energy is emitted as heat. So, whenever the free electron comes the fill up with the hole, the process called recombination takes place.

Recombination energy and carrier life time

We know that in an intrinsic semiconductor new hole electron pair is generated due to thermal agitation. Since the life time of the electrons are limited one, after completing the life time, the electrons in the conduction band fall back to the valence band.

Whenever electrons in the conduction band travels to the valence band, electron combines with hole with the release of excess of energy, called recombination energy. This energy is emitted in the form of heat.

It is clear that on the average, an electron will exist for a life time and a hole will exist for a life time, seconds before recombination.

The time for which, on an average, a charge carriers will exist before recombination with a carrier of opposite charge is called carrier life time.