Photoconductivity of CdS-Type Photoconductors in the Vicinity of the Absorption Edge

The marked drop in photoconductivity on the high-energy side of the absorption edge in high quantum-yield photoconductors cannot be explained satisfactorily in terms of the models based on the trapping of the free holes produced in band-to-band excitations. In the model described, the photocurrent is assumed to arise mainly through the excitation of electrons from discrete levels very close to the valence band. The trapped holes formed in this process are in an excited state and drop to a ground state further away from the valence band, where they are relatively stable against thermal ionization and give rise to high quantum yields. This model for the photoconductivity mechanism appears to explain a large variety of experimentally observed characteristics.