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 $\frac{1}{f}$ trend when the electron quasi-Fermi level is not located near a discrete trap. The $\frac{1}{f}$ 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 $\left(\frac{1}{\omega }\right)invtan\omega \tau$, where $\tau$ is determined by the low-frequency turnover of the $\frac{1}{f}$ trapping noise. From these experimental values of $\tau$, 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 $\frac{1}{f}$ spectra are thus related to the discrete traps, rather than to the position of the electron quasi-Fermi level directly, it appears that the $\frac{1}{f}$ 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.