Why do different semiconductor photoconductors respond to different frequencies of light?

Different semiconductor photoconductors respond to specific frequencies of light because of their unique energy gap ($E_g$), which is the energy difference between the valence band and the conduction band.12

For a semiconductor to exhibit photoconductivity, an incident photon must possess energy ($h\nu$) greater than or equal to the material's specific band gap. When a photon with sufficient energy strikes the material, it excites an electron from the valence band to the conduction band, creating a hole-electron pair that increases the material's conductivity.1

Because the value of $E_g$ varies between different materials, the threshold frequency required to trigger this internal photoelectric effect changes accordingly:1

• Lead sulfide has a small band gap, resulting in a threshold frequency in the infrared range.1
• Silicon has a band gap of approximately 1.1 eV, with a threshold wavelength in the infrared (about 1,100 nanometres).1
• Zinc oxide has a larger band gap, placing its threshold in the ultraviolet range.1

Consequently, the specific electronic structure of each semiconductor dictates which portion of the electromagnetic spectrum it can detect.1