Direct and Indirect band gaps - Engineering Physics


Direct and Indirect bandgap in semiconductor

Direct Bandgap semiconductors – 

In direct bandgap semiconductor, the bottom of the conduction band and top of the valence band lies at the same value of K. In this, electron can directly excite or de-excite by the absorption or emission of photon and there is no phonon involvement in the process of excitation and de-excitation.

If a photon incident of energy,, there is absorption then electron of valence band will absorb this energy and excite to the conduction band and when it de-excite to valence band, then it will emit some energy i.e.  .

There is no requirement of phonon or lattice in the conservation of energy and momentum. Direct bandgap semiconductors are used in light-emitting applications like LED and LASER. Ex. GaAs, CdS, ZnS, CdSe etc.

Indirect Band Gap semiconductor

In Indirect bandgap semiconductor, top of the valence band and bottom of the conduction band lies at different values of K. If an electron goes from the top of the valence band to the bottom of the conduction band, it has to change its energy as well as wave-vector K.

For momentumand energy conservation, there is the involvement of phonon in the conservation process. If there is de-excitation of the electron, then not all the energy will be emitted in the form of the photon but some energy is emitted in the form of phonons i.e. some part is transferred to the lattice, and the lattice will vibrate and generate heat. So indirect bandgap semiconductor bandgap semiconductor is not suitable for light emission. Ex. Si, Ge, GaP SiC, etc.

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