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.