4 Physical mechanisms: Absorption, photoconductivity, photon emission
4.2 Photoconductivity and photoelectric effect

Photoconductivity is an optoelectronic phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation such as light.

Photoelectric effect: Many metals emit electrons when light shines upon them. In the photoemission process, if an electron within some material absorbs the energy of one photon and acquires more energy than the work function of the material, it is ejected.

Einstein was awarded the Nobel Prize in 1921 for his research on the photoelectric effect. The energy required to remove an electron from the material is called the work function of the metal, ϕ.

Photon emission: When an electron falls into a lower energy level and meets a hole, it releases energy in the form of a photon. The wavelength of the light depends on the band gap of the semiconductor material. Light is emitted in multiples of a certain minimum energy unit. The size of that unit is the photon energy.

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Fig. 7. Photon emission.

The photon energy is: (013), being c the speed of light in vacuum.

Calculate the range of wavelengths not absorbed by Germanium: Ge, by taking into account the bandgap of Ge = 0.66 eV.

SOLUTION

Light absorption in a semiconductor creates pairs e-h+ when the energy of incident photons is greater than the band gap of the material, Eg. In case of Ge, the minimum value of energy for photons to get absorbed will be:

(014). So the photons having wavelengths: (015) will be absorbed by the semiconductor.

Considering hc = 1.24 eVμm. The maximum value of photons wavelength to generate pairs e-h+ in Ge is: λ < 1878 nm.

All physics effects described in this chapter have specific application in optoelectronic technologies as well as in other related physical sciences.