Question 3-19C1

Photoconductive materials, often semiconductors like cadmium sulfide (CdS), are engineered to change their electrical resistance in response to light. When photons of light with sufficient energy strike the material, they are absorbed. This absorption excites electrons, providing them enough energy to jump from the valence band to the conduction band. Once in the conduction band, these electrons become mobile charge carriers. The "holes" left behind in the valence band also act as charge carriers. The significant increase in the number of free charge carriers (electrons and holes) within the material allows current to flow more easily. Therefore, the material's ability to conduct electricity improves, meaning its conductivity **increases**. * **A) It increases:** This is correct because light generates additional free charge carriers. * **B) It decreases:** This would imply light inhibits charge movement, which is not what happens in photoconductive materials. * **C) It stays the same:** This is incorrect; the fundamental property of these materials is a change in conductivity with light. * **D) It becomes temperature dependent:** While all semiconductors are temperature-dependent, the primary and direct effect of light is to change the conductivity itself, not merely make it dependent on temperature.