With a large density of impurities or other lattice defects, the carrier transport deviates substantially from the classical transport within the band. It is carried within energy ranges (within the bandgap), which are determined by the defect structure. Heavy doping produces predominant defect levels split into two impurity bands. Below a density to permit sufficient tunneling, carrier transport requires excitation into the conduction band; at higher defect density, a diffusive transport within the upper impurity band becomes possible. At further increased defect density, metallic conductivity within the then unsplit impurity band occurs.
In amorphous semiconductors, tunneling-induced carrier transport can take place within the tail of states, which extend from the conduction or valence band into the bandgap. Major carrier transport starts at an energy referred to as the mobility edge. With statistically distributed defects, only some volume elements may become conductive. These volume elements widen at increasing temperature, eventually providing an uninterrupted percolation path through a highly doped or disordered semiconductor with a density-related threshold of conduction.
Conductance in organic semiconductors is governed by static and dynamic disorder. Band conductance in small-molecule crystals shows a decreasing carrier mobility at increased temperature with a power law similar to that of inorganic semiconductors. Small-molecule or polymer semiconductors with dominating static disorder show hopping conductance with a typically low but increasing mobility at higher temperatures.
Amorphous semiconductor Band conductance Dispersive transport Heavy doping Hopping conduction Hopping mobility Impurity band Mobility edge Organic semiconductor Percolation Phonon-activated conduction Tunneling-induced transport Variable-range hopping
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