Fluctuations in CdS Due to Shallow Traps

The observed current noise of a lightly doped cadmium sulfide single crystal under uniform illumination is quantitatively explained by application of the generation-recombination theorem to retrapping effects. The results yield a frequency factor of ${10}^9$ ${\sec }^{-1\mathrm{}}$ for a discrete set of traps located 0.4 ev below the conduction band and ${10}^8$ ${\sec }^{-1\mathrm{}}$ for traps distributed between 0.3 to 0.5 ev. When the quasi-Fermi level is near the discrete states, the current noise spectra show a relaxation component characteristic of generation and recombination involving these levels.