Question

list the different types of photoexcitation of charge carriers and corrosponding relaxation mechanism in doped samples.

Answer 1

In semiconductors, a doped sample can be of two types, it can be doped with acceptors or with donors (leading to n-type semiconductor).

Mechanism 1

When light is incident upon n-type semiconductor, as in it,donor level is near the conduction band; electrons will jump to valence band leaving behind holes giving rise to flow of current leading to conductivity.

Similarly ,in case of p type semiconductors; acceptor level is near the valence band. On photo-excitation electrons from valence band jumps to it leading to conductivity.

Mechanism 2

In case if incident energy is more than band gap , electrons can directly jump from valence to conduction band giving rise to inter band transition.

Mechanism 3(relaxation)

If incident energy is much more than band gap then electron is unstable in conduction band . Thermalization occurs and it  comes down in the conduction band through intraband transition. This is hot electron concept.Heat is generated in it.

In case of defects, electrons can get trapped in these upon photo excitation.

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