An experimental study on PbS photodetectors—of the Ektron type—is reported. The results show that the ratio of the responsivity over the conductance multiplied by the time constant is temperature-independent between 300° K and 80° K. This is shown to be consistent with the experimental results obtained on different types of PbS cells and explained by either the number or the barrier theory. The actual contradictions are removed by a new model to be proposed in Part II of this study.

The experimental properties analyzed in Part I of this study are compared with a theoretical analysis of carrier recombination through multiple levels already published by the author. This suggests the possibility of a model of the number theory type. A trap for the electron carriers is located at 0.13 ev below the conduction band and a recombination level at 0.10 ev below the trap level. The Fermi and the recombination levels are coincident. The number of trapping centers is about 1018cm−3, and the number of recombination centers 2 × 1017cm−3. Almost all the experimental evidence used in support of both the barrier and the number theory can be verified by the new model, and the previous contradictions are removed. The behavior of the photodetector under various background illumination levels is also explained.