Tunable oscillatory modes of electric-field domains in doped semiconductor superlattices are reported. The experimental investigations demonstrate the realization of tunable, GHz frequencies in GaAs-AlAs superlattices covering the temperature region from 5 to 300 K. The orgin of the tunable oscillatory modes is determined using an analytical and a numerical modeling of the dynamics of domain formation. Three different oscillatory modes are found. Their presence depends on the actual shape of the drift velocity curve, the doping density, the boundary condition, and the length of the superlattice. For most bias regions, the self-sustained oscillations are due to the formation, motion, and recycling of the domain boundary inside the superlattice. For some biases, the strengths of the low and high field domain change periodically in time with the domain boundary being pinned within a few quantum wells. The dependency of the frequency on the coupling leads to the prediction of a new type of tunable GHz oscillator based on semiconductor superlattices.Comment: Tex file (20 pages) and 16 postscript figure

A semiconductor superlattice represents an ideal one-dimensional nonlinear dynamical system with a large number of degrees of freedom. The effective nonlinear coupling originates from sequential resonant tunneling between adjacent wells. We have observed spontaneous chaotic and periodic current oscillations in a doped GaAs͞AlAs superlattice by changing only the applied bias. When the system is driven with an incommensurate sinusoidal voltage for a fixed bias, transitions between synchronization and chaos are observed via pattern forming bifurcations. A driving signal of sufficiently large amplitude can suppress the occurrence of chaos and produce a synchronized oscillation mode with a subharmonic of the driving frequency.[S0031-9007(96)01318-X]

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Electric-field domain formation in doped semiconductor superlattices leads to sharp discontinuities in the current-voltage (I-V) characteristic. The successive expansion of the high-field region with increasing bias voltage through the periodic heterostructure manifests itself in a regular sequence of stable current branches. The current shows a complex hysteretic behavior. We observe two, three, and more stable current levels for fixed bias voltages. Calculations of the I-V characteristic based on a microscopic model support the experimentally observed multistability.

Self-sustained current oscillations have been found in doped GaAs-AlAs superlattices and are investigated experimentally and theoretically. The electric-field distribution in doped superlattices becomes unstable if the carrier density is not sufficiently large to form stable domains. The instability results in self-oscillations of the current, which are due to an oscillatory motion of the domain boundary in the superlattice. A discrete drift model taking into account several regions of negative differential velocity and a large electron density clearly establishes the origin of these oscillations.