An important technique for measuring the band gap energy of a semiconductor is the absorption of incident photons by the material.
Optical Absorption of photon:
- In this experiment photons of selected wavelengths are directed at the sample, and relative transmission of the various photons is observed.
- Since photons with energies greater than the band gap energy are absorbed while photons with energies less than the band gap are transmitted, this experiment gives an accurate measure of the band gap energy
- It is apparent that a photon with energy hv > Eg can be absorbed in a semiconductor that is shown in figure.
- Since the valence band contains many electrons and the conduction band has many empty states into which the electrons may be excited, the probability of photon absorption is high.
- As Fig. indicates an electron excited to the conduction band by optical absorption may initially have more energy than is common for conduction band electrons (almost all electrons are near Ec unless the sample is very heavily doped).
- Thus the excited electron loses energy to the lattice in scattering events until its velocity reaches the thermal equilibrium velocity of other conduction band electrons.
- The electron and hole created by this absorption process are excess carriers; since they are out of balance with their environment, they must eventually recombine.
- While the excess carriers exist in their respective bands, however, they are free to contribute to the conductivity of the material.
- A photon with energy less than Eg is unable to excite an electron from the valence band to the conduction band.
- Thus in a pure semiconductor, there is negligible absorption of photons with hv < Eg. This explains why some materials are transparent in certain wavelength ranges.
- We are able to "see through" certain insulators, such as a good NaCl crystal, because a large energy gap containing no electron states exists in the material.
- If the band gap is about 2 eV wide, only long wavelengths (infrared) and the red part of the visible spectrum are transmitted; on the other hand, a band gap of about 3 eV allows infrared and the entire visible spectrum to be transmitted.