What is the difference between the external and internal photoelectric effects?

The photoelectric effect describes the release of electrically charged particles from or within a material upon the absorption of electromagnetic radiation. The distinction between the external and internal effects lies in whether the particles are ejected from the material or merely redistributed within it.1

External Photoelectric Effect

Often referred to simply as the photoelectric effect, this process occurs when incident photons possess sufficient energy to overcome the work function of a material, typically a metal. As a result, electrons are completely ejected from the surface of the material into the surrounding space.21

Internal Photoelectric Effect

This process occurs within materials such as semiconductors. When photons with energy exceeding the material's energy gap are absorbed, electrons are excited from the valence band to the conduction band. This creates hole-electron pairs that remain within the material, significantly increasing its electrical conductivity—a phenomenon known as photoconductivity.23