When light propagates through a material there is a conversion of part the photons energy to other forms of energy (e.g. Heat). This lost energy is absorbed by the material. Electrons of atoms can move to the higher-energy states and be excited from the VB (valence band) to the CB (conduction band) by absorption of the energy of photons and pairs e–-h+ (electron-hole) are created by this mechanism.
The most important process of light absorption in a semiconductor is the creation of those pairs e–-h+. Each absorbed photon causes a transition from the valence band to the conduction band. A photon is absorbed by a semiconductor if the photon energy is greater than the band gap of the material, Eg.
The band gap, Eg, generally refers to the energy difference, in eV (electron volts) between the top of the VB and the bottom of the CB in insulators and semiconductors. The electron affinity of a semiconductor,χ, is the width of the CB in eV. The Fermi energy, EF, indicates the highest energy states occupied energy at 0 K. Energy states above EF are empty up to the vacuum level.
Eg = Ec – Ev
where Ec and Ev are the energy corresponding to the top of the VB and the bottom of the CB. Fig. 6 shows the absorption mechanism and the energy band diagram.
Semiconductor |
Eg (eV) |
χ(eV) |
Silicon : Si |
1.11 |
4.05 |
Gallium Arsenide :GaAs |
1.42 |
4.07 |
Germanium: Ge |
0.66 |
4.13 |
Indium Phosphide : InP |
1.35 |
4.5 |
Gallium Phosphide : GaP |
2.26 |
3.8 |
For each wavelength, λ, of the incident beam of light, Io, passing through the material, the intensity of the light beam, I, is attenuated by scattering and absorption mechanisms. Lamber’s law defines transmission and absorption as follows
I = Io·e–αL
where α is the absorption coefficient; α (m-1) is a function of λ.