  Noise-enhanced chaos in a doped, weakly coupled GaAs/Al_{0.45}Ga_{0.55}As
superlattice has been observed at room temperature in experiments as well as in
the results of the simulation of nonlinear transport based on a discrete
tunneling model. When external noise is added, both the measured and simulated
current-versus-time traces contain irregularly spaced spikes for particular
applied voltages, which separate a regime of periodic current oscillations from
a region of no current oscillations at all. In the voltage region without
current oscillations, the electric field profile consist of a low-field domain
near the emitter contact separated by a domain wall consisting of a charge
accumulation layer from a high-field regime closer to the collector contact.
With increasing noise amplitude, spontaneous chaotic current oscillations
appear over a wider bias voltage range. For these bias voltages, the domain
boundary between the two electric-field domains becomes unstable, and very
small current or voltage fluctuations can trigger the domain boundary to move
toward the collector and induce chaotic current spikes. The experimentally
observed features are qualitatively very well reproduced by the simulations.
Increased noise can consequently enhance chaotic current oscillations in
semiconductor superlattices.
