photoconductive

Photoconductivity is the increase in a material's electrical conductivity when it is illuminated. In most semiconductors, photons with energy at least equal to the bandgap generate electron–hole pairs, raising the density of mobile charge carriers and thus the conductivity. Some insulators or defect-rich materials also exhibit photoconductivity through trap-assisted mechanisms. In darkness, the conductivity is typically lower and the photocurrent ceases when illumination stops.

Photoconductive devices rely on a photosensitive layer and electrical contacts. Illumination raises carrier concentration, producing a measurable current under an applied bias. The current generally rises with light intensity and falls after illumination ends with a characteristic decay determined by carrier lifetimes and trap dynamics. Materials with shallow traps tend to respond quickly, while those with deep traps can show persistent photoconductivity.

Materials commonly used include amorphous selenium, cadmium sulfide, lead sulfide, silicon, and gallium arsenide; organic photoconductors are also employed in some imaging and sensing applications. Photoconductive detectors are used in cameras, spectrometers, and optical communication systems, while photoconductive cells or light-dependent resistors operate as light-controlled resistors for sensing and control tasks. In xerography, a photoconductive layer is charged in the dark and exposed to light to form a latent electrostatic image.

Important performance parameters are spectral responsivity, linearity with light intensity, response time, and dark current. The response depends on bandgap, defect levels, temperature, and device geometry. Photoconductivity provides a simple mechanism for light detection and imaging but can be limited by noise, persistence, or high dark conduction in some materials.