Single-electron current count checked
Single-electron pumps allow a current to be generated by trapping and transporting single electrons in a controlled manner. PTB has now succeeded in verifying the trapping statistics of a semiconductor single-electron pump for the first time by detecting single electrons with the aid of a special detector circuit.
In the future SI system, the physical units are to be defined by means of elementary fundamental constants such as Planck’s constant h or the charge of an electron e. The base unit of electric current, the ampere, can then be realized via a so-called “singleelectron pump”. The single-electron pump consists of a microscopic semiconducting island with two current leads. In pumping operation, first an electron coming from the current lead on the left is loaded onto the island and then released into the other current lead. If this procedure is repeated periodically at a clock frequency ƒ, a current I = eƒ is generated. The current is then only determined by the fundamental constant of the charge of an electron e, and the clock frequency ƒ. At present, semiconductor- based single-electron pumps are considered the most promising candidates for the future realization of the ampere.
In the past, to characterize such a single-electron pump, the pump had to be operated continuously with a given frequency f, and the current generated thereby had to be measured as precisely as possible. Such a measurement, however, always implies an averaging over numerous clock cycles, so that the data concerning single, rarely occurring pump errors are eventually lost. The actual number of these pump errors is, however, of vital importance for metrological applications. In the year under report, PTB was able, for the first time, to integrate and test a detector circuit together with a single-electron pump on one chip (see figure). The detector circuit allowed single errors of such pumps to be detected and analyzed.
The detector circuit is based on so-called “singleelectron detectors”. These detectors react so sensitively to electric charges that they can even definitely detect the only electron that is trapped and transported by the pump per clock cycle. With the aid of this detection method, the error rate of the pump has now been measured precisely as a function of various external parameters. Excellent agreement was yielded when comparing the measured error rate with theoretical predictions; this confirmed the validity of the model used. In addition, it turned out that under the given conditions of measurement, the thermal distribution of the electrons on the island did not have a significant influence on the error rate. These results are a decisive step towards the future development of a semiconductor-based single-electron current standard.