Investigation of the Photoconductive Effect in Lead Sulfide Films Using Hall and Resistivity Measurements

A long-standing question in the theory of photoconductivity in semiconductor films is whether the change in conductivity can be attributed completely to a change in carrier density, or whether current amplification occurs through modulation of barrier potentials. This question is studied in chemically deposited films of lead sulfide by measuring the fractional change in Hall coefficient, $\frac{\Delta R_H}{R_H}$, and the fractional change in resistivity, $\frac{\Delta \rho }{\rho }$, under illumination. The measurements were made over the temperature range -41°C to +31°C. It was found that $\frac{\Delta R_H}{R_H}=\left(\frac{\Delta \rho }{\rho }\right)(1\mathrm{}\pm 0.06)$ in this temperature range. Expressed in terms of the Hall mobility, ${\mu }_H=\frac{R_H}{\rho }$, the result is that $\frac{\Delta {\mu }_H}{{\mu }_H}=0\mathrm{}\pm 0.06\frac{\Delta \sigma }{\sigma }$. A model of photoconductivity is considered, which assumes that the primary photoeffect is a change in the density of majority carriers in the PbS crystallites and that secondary barrier amplification effects can occur by trapping of minority carriers at the intercrystalline barriers. Application of our result, $\frac{\Delta {\mu }_H}{{\mu }_H}=0\mathrm{}$, to this model leads to the conclusion that photoconductivity in chemically deposited lead sulfide films can be accounted for by changes in majority carrier concentration in the PbS crystallites; no barrier amplification occurs.