Why is lead sulfide useful for detecting infrared radiation?

Lead sulfide is highly effective for detecting infrared radiation due to its specific electronic properties related to the photoelectric effect and photoconductivity.

In semiconductors like lead sulfide, electrical conductivity increases when the material absorbs photons with sufficient energy to excite electrons across the band gap—the energy difference between the valence band and the conduction band. While many materials require higher energy (such as visible light) to trigger this process, lead sulfide has a narrow band gap of approximately 0.41 electron volts.1

This low energy requirement allows lead sulfide to respond to the lower-energy photons characteristic of infrared radiation. When these photons strike the material, they provide enough energy to move electrons into the conduction band, thereby increasing the material's electrical conductivity. This sensitivity makes lead sulfide a valuable component in photoconductive cells used for various applications, including military infrared detection systems.12