The xerographic (dry writing) process was invented in 1938...

The xerographic (dry writing) process was invented in 1938 by C. Carlson. In xerography, an image is produced on a photoconductor by exposing it to light. Selenium is commonly used, since its conductivity increases three orders of magnitude upon exposure to light in the range from 400 to $500 \mathrm{~nm}$. What color light should be used to cause selenium to become conductive? (See Figure 7.2.)

The xerographic (dry writing) process was invented in 1938 by
C. Carlson. In xerography, an image is produced on a photoconductor by exposing it to light. Selenium is commonly used, since its conductivity increases three orders of magnitude upon exposure to light in the range from 400 to $500 \mathrm{~nm}$. What color light should be used to cause selenium to become conductive? (See Figure 7.2.)

Key Concepts

Photoconductivity

This is the phenomenon where a material's electrical conductivity increases when exposed to light. When photons are absorbed, they generate charge carriers (electrons and holes), allowing the material to conduct electricity more efficiently. This principle is fundamental in devices that convert light into electrical signals.

Xerography

Xerography is a dry photocopying process that uses a photoconductive material to form an image. The process involves charging a photoconductor, exposing it to light to create a latent image, and then using toner to produce a visible copy. Understanding xerography requires a grasp of how light exposure affects the electrical properties of photoconductive materials.

Semiconductor Band Gap

The band gap is the energy difference between the valence band and the conduction band in a semiconductor. When light with photon energy equal to or greater than this gap is absorbed, electrons can be excited into the conduction band, leading to increased conductivity. This concept explains why certain wavelengths of light are effective in triggering conductivity in semiconductor photoconductors.

Spectral Sensitivity

Spectral sensitivity refers to a material's response to different wavelengths of light. For photoconductive materials, only light within a certain range of wavelengths (or energies) can generate sufficient charge carriers to enhance conductivity. This concept is essential to understanding how the color of light, determined by its wavelength, affects the performance of photoconductive devices.

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