Electron and Hole Transport in CdS Crystals

Abstract

Fast pulse techniques have been used to study the drift mobility of both electrons and holes in undoped CdS crystals of high resistivity over the temperature range from 500 °K to 80 °K. A short electron pulse generated free carriers near the top electrode and measurements of the transit time in a pulsed applied field led to a value for the drift mobility. The method is sensitive to the injection of additional carriers and the choice of electrodes is discussed. In the temperature range above about 160 °K the electron drift mobility μ_e is in good agreement with Hall mobility values. Below this temperature a transition takes place to a charge transport predominantly controlled by a level of states close to the conduction band. The depth of the centres depends on their density; at infinite dilution it is 0.049 ev. Calculated (μ_e, 1/T) curves agree well with the experimental values. It is shown that all the published electron mobility data lie suprisingly close to a mobility curve determined by acoustic mode scattering (μ ∝ T^-3/2) from 700 °K to about 80 °K. The experimental method provides the first direct evidence for the transport of holes in CdS. At room temperature the hole drift mobility μ_h has a mean value of 15 cm² sec^-1 v^-1 and the hole lifetime lies between 1 × 10^-7 sec and 3 × 10^-7 sec. In the lower temperature range all log μ_h against 1/T curves decrease linearly with an activation energy of 0.019 + 0.002 ev. An attempt has been made to interpret the results in terms of a model of hole transport in the upper two valence bands of CdS.