Semiconductor superlattices: a model system for nonlinear transport

Electric transport in semiconductor superlattices is dominated by pronounced negative differential conductivity. In this report, the standard transport theories for superlattices, i.e. miniband conduction, Wannier-Stark hopping, and sequential tunneling, are reviewed in detail. Their relation to each other is clarified by a comparison with a quantum transport model based on nonequilibrium. Green functions. It is demonstrated that the occurrence of negative differential conductivity causes inhomogeneous electric field distributions, yielding either a characteristic sawtooth shape of the current-voltage characteristic or self-sustained current oscillations. An additional ac-voltage in the THz range is included in the theory as well. The results display absolute negative conductance, photon-assisted tunneling, the possibility of gain, and a negative tunneling capacitance. (C) 2002 Elsevier Science B.V. All rights reserved.