Fermi-level effects ina-Si:H photoconductivity

Fermi-level—position effects on photoconductivity in $a$-Si:H were studied using metal-oxide-semiconductor field-effect transistor devices. When the Fermi level shifts toward the conduction band as a result of field effect, the photoconductive current increases monotonically without saturation and the exponent of its illumination intensity dependence is gradually reduced from 0.9 to 0.4. These features can be explained if photoexcited electrons are distributed among extended states and localized states and if the nonradiative-recombination rate through recombination centers is controlled by the free-hole—capture rate of the recombination centers.