Current Noise and Distributed Traps in Cadmium Sulfide

The high-frequency portion of the current noise spectrum observed in lightly doped CdS single crystals under uniform 5200 A illumination is characteristic of electron trapping transitions to shallow levels. In many crystals the noise spectra have a characteristic 1/f trend when the electron quasi-Fermi level is not located near a discrete trap. The 1/f trapping noise observed in one crystal at temperatures of 10, 27, and 52^∘{}C and for 20 different positions of the electron quasi-Fermi level between 0.5 and 0.3 ev below the conduction band can be represented by a single expression of the form (1ω)invtanωτ, where τ is determined by the low-frequency turnover of the 1/f trapping noise. From these experimental values of τ, trap depths are calculated which are in good agreement with the positions of discrete trapping levels determined from other measurements. Since the low-frequency turnovers of the 1/f spectra are thus related to the discrete traps, rather than to the position of the electron quasi-Fermi level directly, it appears that the 1/f noise may not be associated with a postulated continuous distribution of traps in energy, but rather with a dispersion of capture and release times into the discrete traps.