Electron transport and shot noise in ultrashort single-barrier semiconductor heterostructures

We present a theoretical study of electron transport and shot noise in ultrashort single barrier semiconductor structures. Calculations are applied to a simple GaAs semiconductor model in the presence of ballistic and thermalized carriers. The coupling between space charge and the dependence of the transmission coefficient on energy is found to provide the positive feedback that enhances shot noise and ultimately leads to a current instability of S type. When the strength of this feedback is weak, shot-noise suppression is observed. The occurrence of enhanced shot noise is explained in terms of a negative lifetime related to carrier escape through the collector contact. The model also predicts shot-noise enhancement in single barrier structures with constant transparency in the region of current saturation. Theoretical results are in qualitative agreement with existing current-voltage experiments and confirm recent Monte Carlo simulations evidencing shot-noise enhancement in GaAs/AlGaAs semiconductor heterostructures. Shot-noise enhancement is found to be a precursor indicator that the device is approaching an instability regime in analogy with the case of phase transitions.