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According to a barrier photoconductance theory recently developed, two current noise sources can be envisaged, related respectively to the fluctuation of the barrier height (photoinduced noise component) and to the trapping–detrapping processes in shallow states within the photoconducting material (g–r and 1/f noise component). It is shown that the first noise component, which can be separated by the second one, gives information on the energy gap and on the photoionization cross section of the deep energy levels of the photoconducting material. In particular for thin films it will be shown that the photoionization cross section of the deep energy levels varies inversely to the total number of photons impinging on the photoconductor and proportionally to the noise power spectral density as the photon energy changes. Such relationship suggests that the wavelength dependence of the noise power spectrum provides the correction to be taken into account if the constant photocurrent method is used and the variations of the minority carrier lifetime with the photon energy cannot be disregarded. Results concerning the energy gap and its temperature dependence, in the interval ranging from the room temperature to 200 K, are reported and compared with the results found in the literature for CdS based samples. The photoionization cross section of the deep centers as a function of the photon energy, at room temperature, is reported for the same sample. Such quantities have been obtained by measuring the spectral density of the photoinduced noise vs wavelength at constant photoconductance value. As for the other kinds of noise spectroscopy, the main advantage of the present method is to work out in the operative condition of the semiconductor device. © 1996 American Institute of Physics.