Carrier transport and mobility

Conduction can also operate in the band tails below the mobility edge when such tails are wide and have a sufficiently high density of states to allow rapid hopping of a carrier from one site to another via thermally-activated processes. In this case the mobility of a carrier depends exponentially upon the amount of energy it must gain to leave a given state and hop to the next. Thus, it was proposed that the mobility will have the form [5] 

A temperature dependence of the conductivity scaling as an exponential of is a standard signature of variable-range phonon-assisted turmeling conduction. A similar behavior is found for conduction in bands of defect states in crystalline semiconductors. 

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