UNIT-III

Optoelectronics

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Generation and recombination of carriers

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Generation and recombination of carriers

• Generation of carriers (free electrons and holes)
• The process by which free electrons and holes are generated in pair is called generation of carriers.
• When electrons in a valence band get enough energy, then they will absorb this energy and jumps into the conduction band. The electron which is jumped into a conduction band is called free electron and the place from where electron left is called hole. Likewise, two type of charge carriers (free electrons and holes) gets generated.

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• Recombination of carriers (free electrons and holes)
• The process by which free electrons and the holes get eliminated is called recombination of carriers. When free electron in the conduction band falls in to a hole in the valence band, then the free electron and hole gets eliminated.

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• When sufficient energy is given to the semiconducting material, covalent bonds are broken and a pair of electron and hole is generated.
• The electrons are raised from valence band to conduction band. Holes remain in the valence band.
• The process of generation can occur only when the energy given to the semiconducting material is greater than its forbidden energy gap.
• Recombination is the process where an electron from the conduction band recombines with a hole in the valence band. This is called band to band recombination and in the process a photon is released.

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• Recombination can be classified into following
• Direct recombination
• Indirect recombination

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� Direct band gap material�

The maximum of VB and the minimum of CB exists at the same value of wave number Such semiconductors are called direct

band gap material.

✔ In this type during the recombination of

holes and electron, a photon of light is

released. This process is known as radiative

recombination. Also called as spontaneous

emission.

✔ It is more effective, because the direction of

motion of electron remains unchanged.

✔ In this type of semiconductors life

timeof charge carrier is very less.

✔ Electron – hole pair can be easily generated because it requires less momentum.

✔ Recombination probability is much high.

✔ These are used to fabricate LEDs and laser diodes.

✔ These are mostly from the compound semiconductors.

Ex: InP, GaAs …….

Indirect band gap material

INDIRECT BAND GAP SEMICONDUCTOR:

• The maximum value of VB and the minimum value of CB exists at the different values of wave number . Such semiconductors are called Indirect Band Gap Semiconductor.
• In this type, heat energy is produced during the recombination of holes and electrons. This process is known as non radiative recombination.
• It is less efficient, because the direction of motion of electron changed.
• In this type of semiconductors life time of charge carrier is high.
• The incident process is slower, because it requires an electron, hole and phonon for interaction.
• Recombination occurs through some defect states.
• Due to the longer life time of charge carriers, these are used to amplify the signals as in the case of diodes and transistors.
•  These are mostly from the elemental semiconductors.
• Ex:Si ,Ge