Absorption and emission of light in semiconductors

ABSORPTION AND EMISSION OF LIGHT IN SEMICONDUCTORS

In semiconductors, light photons is absorbed in several ways. In intrinsic (pure) semiconductors such as Si, Ge and GaAs, light photons is absorbed to create electron-hole pairs.

This absorption causes electrons to jump across the energy band gap from the valence band to the conduction band as shown in fig. 4.2(a).

This transition occurs ie., the excitation of electrons due to absorption can take place if the photon energy is greater than that of the band gap E_g that is if

hv > E_g …. (1)

where h - Planck's constant

v - Frequency of the light photon.

In terms of wavelength λ

hc / λ > E_g  ...(2)

The maximum wavelength for visible light λ_max is about 0.7 μm. Therefore, the minimum band gap energy E_g(min) for which there is absorption of visible light is given by

E_g(min) = hc / λ_max

Substituting the corresponding values, we have

The result indicates that all visible light is absorbed by those semiconductors having band gap energies less than about 1.8 eV. Thus, these semiconductors are opaque.

In extrinsic (impure) semiconductors, the presence of acceptor and donor impurities creates new energy levels namely acceptor level (E) (p-type semiconductor) and donor level (E) (n-type semiconductor) as shown in fig. 4.4 (b) and (c). These impurity levels lie within the band gap of the material.

Light radiation of specific wavelength may be absorbed as a result of electron transitions from or to these impurity levels within the band gap.