Electron and Hole Transport in CdS Crystals

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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 μe 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 (μe, 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 μh 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 μh 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.