Current Saturation and Trap-Controlled Electron Drift Mobility in Photoconductive CdS

Current saturation due to acoustic oscillations in CdS is observed, both in a transverse mode and in a longitudinal mode. The most pronounced saturation occurred in the transverse mode, although the applied dc field was parallel to the $c$ axis, and one should expect saturation mainly in the longitudinal mode. A method for determining the threshold field for oscillation, utilizing the buildup time for current saturation under applied pulsed dc electric field, is discussed. The threshold field is used to determine the electron drift mobility for photoconducting CdS in the temperature range from 204 to 438°K. The temperature dependence of the mobility can be described as a combination of scattering from lattice vibration and trapping from two impurity levels, ${\epsilon }_1=0.02\mathrm{}$ eV with density $N_1=6\mathrm{}\times {10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$ and ${\epsilon }_2=0.1\mathrm{}$ eV with density $N_2=8\mathrm{}\times {10}^{16}$ ${\mathrm{cm}}^{-3\mathrm{}}$, and is given by ${\mu }_d=\frac{1.28\times {10}^6{T}^{-\frac{3}{2}}}{1+1420T^{-\frac{3}{2}}{e}^{\frac{0.02}{\mathrm{kT}}}+189\mathrm{}{T}^{-\frac{3}{2}}{e}^{\frac{0.1}{\mathrm{kt}}}}\frac{{\mathrm{cm}}^2}{V sec}.$