The effect of a coherence resonance is observed experimentally in a GaAs/Al_{0.45}Ga_{0.55}As superlattice under dc bias at room temperature, which is driven by noise. For an applied voltage, for which no current self-oscillations are observed, regular current self-oscillations with a frequency of about 82 MHz are induced by exceeding a certain noise amplitude. In addition, a novel kind of a stochastic resonance is identified, which is triggered by the coherence resonance. This stochastic resonance appears when the device is driven by an external ac signal with a frequency, which is relatively close to that of the regular current self-oscillations at the coherence resonance. The intrinsic oscillation mode in the coherence resonance is found to be phase locked by an extremely weak ac signal. It is demonstrated that an excitable superlattice device can be used for the fast detection of weak signals submerged in noise. These results are very well reproduced by results using numerical simulations based on a sequential resonant tunneling model of nonlinear electron transport in semiconductor superlattices.

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.

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