The role of space-charge-limited currents in photoconductivity-controlled devices

The optical exposure required to actuate a photoconductivity-controlled device is shown to be determined by the electric field across the photoconductor and not by the sensitivity of the photoconductor, providing the photoconductive gain is above unity. The exposure is approximately 6 × 10
⁵
KE
photons/cm
²
for a field of
E V
/cm and a dielectric constant
K
. Hence, devices that work at low fields are obviously to be preferred. In order to take advantage of the low field, it is necessary that the photoconductor be able to pass space-charge-limited currents at the same low field. Special trap distributions can lead to optical exposures somewhat lower than the above value.