Carrier Generation and Recombination Processes

CARRIER GENERATION AND RECOMBINATION PROCESSES

The carrier generation is the process whereby electrons and holes are created. The recombination is the process whereby electrons and holes are annihilated.

Carrier Generation

Basically, there are three types of carrier generations. They are

(i) Photogeneration

(ii) Phonon generation

(iii) Impact ionization

(i) Photogeneration

In photogeneration, light of frequency v falls on a semiconductor. Let hv be the energy of light photon greater than the bandgap of the semiconductor.

Figure 4.4 shows the absorption of light energy hv (> Eg).

By absorption of light photon, one electron jumps from valence band to conduction band generating an electron-hole pair.

For different wavelengths of light with different energies (hv2, hv3) it can take an electron in higher conduction band states.

(ii) Phonon Generation

Phonon generation occurs when a semiconductor is under thermal excitation. With increase in temperature of the semiconductor, lattice vibrations increase which give rise to more phonons.

Due to more lattice vibrations, covalent bonds in the semiconductor break down and electron-hole pairs are generated. (Fig. 4.5)

(iii) Impact Ionization

In this process, one energetic charge carrier will create another charge carrier. When a semiconductor is under an electric field, electrons gain energy from the applied electric field and hit other Si-atoms.

In this process, a bond breaks out generating more carriers. For a very high electric field, it results in a avalanche breakdown. (Fig. 4.6)

Recombination

In recombination, a pair of electron and hole gets recombined. When a free electron in the conduction band falls to valence band and recombines with a hole, it becomes a bound electron in valence band.

Recombination occurs in three ways:

(a) Radiative Recombination

(b) Shockley-Read-Hall Recombination

(c) Auger Recombination

(a) Radiative Recombination

It occurs for direct band semiconductors (like GaAs). In this process, electrons from conduction band minimum falls to valence band maximum without changing the momentum. In this process, one photon of energy hv (= Eg) is emitted.

Electrons which are excited to higher energy states in conduction band will come to the conduction band minimum by releasing energy as heat.

Then, from conduction band minimum it falls to valence band maximum emitting light of energy hv = Eg. It is also called direct recombination. (Fig. 4.8)

Shockley-Read-Hall Recombination

In this recombination process, electrons from conduction band minimum come to a defect level intermediate between Ec and Ev by radiating energy as photons or phonons. Then, electron turns from that intermediate level to the valence band.

This type of recombination is basically seen in impure semiconductor which has defect levels. Generally, the defect level lies in the middle of the forbidden gap. (Fig. 4.8)

Auger Recombination

In Auger recombination, three carriers are involved. In this process, an electron and a hole recombine and the energy is given to the third free electron in the conduction band.

Then, the third excited electron comes back to the conduction band edge by emitting energy as heat. Generally, an Auger recombination occurs for heavily doped material. (Fig. 4.9)