  Numerical calculations of anisotropic hopping transport based on the resistor
network model are presented. Conductivity is shown to follow the stretched
exponential dependence on temperature with exponents changing from 1/4 to 1 as
the wave functions become anisotropic and their localization length in the
direction of charge transport decreases. For sufficiently strong anisotropy,
this results in nearest-neighbor hopping at low temperatures due to the
formation of a single conduction path, which adjusts in the planes where the
wave functions overlap strongly. In the perpendicular direction, charge
transport follows variable-range hopping, a behavior that agrees with
experimental data on organic semiconductors.
