Noise, Time-Constant, and Hall Studies on Lead Sulfide Photoconductive Films

Abstract

Measurements of the noise in seven lead sulfide photoconductive films over the frequency range 20-16 000 cps show that the noise consists mainly of a $\frac{1}{f}$ component at frequencies below 100 cps, a generation-recombination component between 100 and 10 000 cps and a Nyquist component at higher frequencies. The data are analyzed to give the magnitude, ${N}_{s0}(\mathrm{G}\ensuremath{-}\mathrm{R})$, and the time constant, ${\ensuremath{\tau}}_{n}$, of the generation-recombination noise. The signal time constant, ${\ensuremath{\tau}}_{s}$, is found by measuring the frequency dependence of the photoconductive response. The product of the majority carrier density and the film thickness, $\mathrm{pd}$, is found from measurement of the Hall voltage. Representative values are ${N}_{s0}(\mathrm{G}\ensuremath{-}\mathrm{R})=4.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ rms noise $\frac{\mathrm{volts}}{[(\mathrm{dc}\mathrm{bias}\mathrm{volt}\ifmmode\times\else\texttimes\fi{}{(\ensuremath{\Delta}f)}^{\frac{1}{2}}]}$, ${\ensuremath{\tau}}_{n}=230$ \ensuremath{\mu}sec, ${\ensuremath{\tau}}_{s}=280$ microseconds, and $pd=3.0\ifmmode\times\else\texttimes\fi{}{10}^{12}/{\mathrm{cm}}^{2}$.From ${\ensuremath{\tau}}_{s}$ and $\mathrm{pd}$, a theoretical value of ${N}_{s0}(\mathrm{G}\ensuremath{-}\mathrm{R})$ is calculated and found to be in good agreement with the experimental value. From this and the reasonable agreement between ${\ensuremath{\tau}}_{s}$ and ${\ensuremath{\tau}}_{n}$ it is concluded that these measurements furnish a verification of the theory of photoconductivity in semiconductor films recently published by one of us. Calculation shows that the generation-recombination noise is mainly due to lattice processes; less than 1% is due to radiation fluctuations. It is also shown that noise can be used in place of Hall measurements to evaluate certain semiconductor parameters.