Synopsis: Transistors amidst heat and noise
At finite temperatures, all electric current comes with noise. In a single-electron transistor—the ubiquitous building block of nanoscale devices, in which a single electrode (“island”) connects two reservoirs (“tunnel junctions”)—the fluctuations in temperature can be particularly large, increasing the noise to the point that it can overwhelm the signal.
Writing in Physical Review Letters, Matti Laakso and Tero Heikkilä at the Aalto University in Finland, and Yuli Nazarov at Delft University of Technology in the Netherlands, study overheating in single-electron transistors. Previous studies largely ignored overheating, treating it as an experimental annoyance; Laakso et al. look in more detail at how overheating influences temperature fluctuations and, in turn, noise.
They study overheating under so-called Coulomb blockade conditions, where the capacitance of the island limits the flow of current. In this case, electron-electron interactions largely determine the temperature of the device. They find that the device’s sensitivity to temperature and also its noise characteristics depend on how electrons in it tunnel, outlining three distinct transport regimes: one where single-electron tunneling dominates, another where cotunneling—the more complicated tunneling of multiple electrons at the same time—dominates, and yet another mixed mode where one sees both types of tunneling. The temperature sensitivity and fluctuation-dependent noise behavior come to a head, as it were, in the crossover zone between the single-electron and mixed-mode regimes.
Laakso et al.’s new way of looking at cotunneling processes should inform further attempts to reduce noise in nanoscale transistors. – Sami Mitra