The band tail density of states distribution

For the exponential tail this expression reduces to a = T/T., but this is the only shape for which a does not depend on and therefore also on time. 

Interpreting the small differences between the data and the predictions of the exponential tail requires caution, because some of the basic assumptions of the model are not soundly based. For example, it is assumed that the transport occurs at a well-defined mobility edge, with a temperature-independent mobility, and both points are open to question. Other effects, such as the internal electric fields of the contacts and deep trapping, lead to distortions of the current pulse from its ideal form. 

The density of states distribution in Fig. 3.16 represents the best current estimates for low defect density a-Si H. The results are sufficiently accurate to use in calculations of the transport and recombination etc., but are continually being refined by new and more accurate experiments. 

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The first term is an energy which is estimated to be close to the mobility edge, so that the second term represents the shift of the transport path below E. Band tail hopping is most significant when the band tail density of states distribution is broad and at low temperatures such that T transport energy moves up to the mobility edge when the temperature is above 2 and the contribution from the band tail hopping is small. 

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