Chapter 21: Problem 24

In your own words, briefly describe the phenomenon of photo conductivity.

Short Answer

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Answer: Photoconductivity of a material is influenced by factors such as the absorption of light energy, the formation of electron-hole pairs, the material's bandgap energy, and the balance between the generation and recombination of electron-hole pairs. Materials with smaller bandgap energies are generally more sensitive to light and have higher photoconductivity. Additionally, longer recombination times lead to more free charge carriers, resulting in higher photoconductivity.

Step by step solution

Introduce Photoconductivity

Photoconductivity is the phenomenon where the electrical conductivity of a material increases in response to light exposure. This occurs because light carries energy, which can be absorbed by the material and create more free charge carriers, thereby increasing its conductivity. Photoconductors are materials which exhibit this property and are often used in various applications such as solar cells and light-sensitive detectors.

Explain the Role of Light in Photoconductivity

Light, being a form of electromagnetic radiation, carries energy in the form of photons. When these photons strike the surface of a material, some of them are absorbed by the material, transferring their energy to the electrons present. This absorption of energy can cause the electrons to gain enough energy to break free from their parent atoms, creating free charge carriers called electron-hole pairs.

Describe the Formation of Electron-Hole Pairs

In semiconductors, electrons require a certain amount of energy, known as the bandgap energy, to be promoted from the valence band to the conduction band. When a photon with an energy equal to or greater than the bandgap energy of the material is absorbed, it can excite an electron from the valence band to the conduction band, leaving behind a positively charged hole in the valence band. These electron-hole pairs are now free to move under the influence of an applied electric field, which contributes to the overall conductivity of the material.

Explain the Relevance of Bandgap Energy

The material's bandgap energy plays a crucial role in determining its photoconductivity. If the photon energy is lower than the bandgap energy, it cannot excite an electron to the conduction band and therefore doesn't contribute to generating electron-hole pairs. Thus, materials with smaller bandgap energies are generally more sensitive to light, as they can absorb a broader range of photon energies.

Discuss the Importance of Recombination in Photoconductivity

After the electron-hole pairs are generated, they can recombine, meaning that the electron falls back into the hole in the valence band, releasing energy in the process. This recombination can be either radiative (emitting a photon) or nonradiative (transferring energy to the surrounding lattice). The balance between the generation and recombination of electron-hole pairs determines the overall photoconductivity of a material. The longer the recombination time, the more free charge carriers will be available, leading to higher photoconductivity.

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In your own words, briefly describe the phenomenon of photo conductivity.

Short Answer

Step by step solution

Introduce Photoconductivity

Explain the Role of Light in Photoconductivity

Describe the Formation of Electron-Hole Pairs

Explain the Relevance of Bandgap Energy

Discuss the Importance of Recombination in Photoconductivity

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