Carrier diffusion in amorphous semiconductors

The increasing use of amorphous semiconducting films in various commercial applications has stimulated considerable interest in the electronic transport properties of such materials. In this review, the authors describe the above phenomenon of 'anomalous dispersion'. Having initially outlined the present view of band structure and carrier transport mechanisms in non-crystalline materials, they continue with a description of the experimental techniques used in the investigation of the drift velocity of excess charge carriers. Details are then provided of the various characteristics associated with both 'conventional' and 'anomalous' dispersion. Models, in which the anomalous behaviour has been associated with both hopping and trap-limited band transport, are reviewed and assessed. Although anomalous dispersion was initially investigated as a phenomenon of interest in its own right, attention has increasingly turned towards the possibility that a detailed examination of the transient behaviour may be of use in determining details of the energy distribution of localised states in disordered films. The authors outline the various techniques which have been suggested and examine their successes and limitations, concluding with an indication of likely future developments and directions of investigations.