Direct and Indirect Band Gap Semiconductor

DIRECT AND INDIRECT BAND GAP SEMICONDUCTORS

Semiconductors can be classified on the basis of the difference in energy level between valence band and conduction band as direct and indirect band gap semiconductors.

That is, if we plot the energy of electron (E) as a function of crystal momentum k, wave vector along the principle direction of crystal we will be able to find two types of graphs as follows.

Direct band gap semiconductor

Electrons in the conduction band occupy available states at bottom of it, whereas holes in the valence band occupy available states at the top positions where they can attain the least energy states. In the case of a direct band gap semiconductor, maximum energy and momentum of electron in valence band is same as minimum energy and momentum of electron in the conduction band which leads to the direct recombination of an electron in the with a hole in the valence band releasing the difference in energy as a photon. E_g = hv

Indirect band gap semiconductor

In the case of an indirect band gap semiconductor, maximum energy and momentum of electron in valence band is different from the minimum energy and momentum of electron in the conduction band which makes the direct recombination of electron and hole is impossible as the transition must conserve energy and momentum and the momentum of the photon is negligibly small to conserve the momentum the system. So, electron first transits to an intermediate level called recombination centre by the release of a phonon and then to the valence band by the release of infrared rays (heat).

Indirect recombination proceeds in a much slower rate than direct recombination as it involves the presence of three entities (electron, phonon and photon).

Examples of direct and indirect band gap semiconductors