Electrical Conductivity of High-Purity Germanium Crystals at Low Temperature*

The temperature dependence of electrical conductivity of single crystalline and polycrystalline high purity germanium (HPGe) samples has been investigated in the temperature range from 7-100K. The conductivity versus inverse of temperature curves for the two sample types have distinct behaviors at different temperature regions and turning points (freeze-out temperatures). Calculations show that the net carrier concentration increases with increasing temperature due to thermal excitation, but it reaches saturation around 40K for the single crystal samples and 70K for the polycrystalline samples. These differences between the single crystal samples and the polycrystalline samples are attributed to trapping and scattering effects of the grain boundaries on the charge carriers. The relevant physical models have been proposed to explain these differences in the conduction behaviors between two kinds of samples. Understanding these properties is important for experiments using HPGe to detect dark matter or neutrinoless double beta decay.