Current noise in barrier photoconducting devices. II. Experiment.

Abstract

Many measurements of photocurrent noise and of other quantities related to the photoconduction processes were performed on CdS based devices to check the theory of current noise, developed on the basis of a barrier-type model of the photoconduction mechanism in the preceding paper. Noise power spectrum measurements have been carried out in a wide range of photon flux densities (8\ifmmode\times\else\texttimes\fi{}${10}^{10}$\ensuremath{\le}${\mathrm{\ensuremath{\varphi}}}_{\mathit{n}\mathit{f}}$\ensuremath{\le}2\ifmmode\times\else\texttimes\fi{}${10}^{13}$ photons ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$) as a function of light wavelength in the range between 400 and 750 nm. The most interesting feature of these results is the abrupt variation of the noise power spectrum occurring in correspondence with the critical wavelength ${\ensuremath{\lambda}}_{\mathrm{gap}}$, even if the device conductance is kept constant by varying the light intensity. Measurements concerning the behavior of the photoconductance, the optical transmittance, and the photoresponsivity versus light intensity and wavelength were also taken and used to determine most of the parameters appearing in the theoretical expression of the noise power spectrum. The relative variations of the shape and amplitude of the noise power spectra with light intensity and wavelength are reproduced by the theory without introduction of free parameters. Also the absolute value of the noise power spectrum at medium to high illumination value (${\mathrm{\ensuremath{\varphi}}}_{\mathit{n}\mathit{f}}$g${10}^{11}$ photons ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$) and at frequencies below 5 kHz, where the photoinduced noise component dominates, is reproduced by the theory without free parameters. Finally, by assuming a suitable value of the parameter ${\mathrm{\ensuremath{\tau}}}_{\mathit{s}}$ appearing in the distribution function of the free-electron lifetimes and representing the inverse of the cutoff angular frequency of the g-r Lorentzian component, the whole set of experimental results is completely fitted by the theory for any value of the light intensity and wavelength, in the explored range.