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.
Böer, K. W., & Pohl, U. W. (2017). Carrier Transport Induced and Controlled by Defects. In Semiconductor Physics (pp. 1–35). Springer International Publishing. https://doi.org/10.1007/978-3-319-06540-3_28-1