Hybrid Metal-Semiconductor Electron Pump for Quantum Metrology

Electron pumps capable of delivering a current higher than 100 pA with sufficient accuracy are likely to become the direct mise en pratique of the possible new quantum definition of the ampere. We present here single-island hybrid metal-semiconductor transistor pumps that combine the simplicity and efficiency of Coulomb blockade in metals with the unsurpassed performances of silicon switches. Robust and simple pumping at 650 MHz and 0.5 K is demonstrated. The pumped current obtained over a voltage-bias range of 1.4 mV corresponds to a relative deviation of $5\times {10}^{-4}$ from the calculated value, well within the $1.5\times {10}^{-3}$ uncertainty of the measurement setup. Multicharge pumping can be performed. The simple design that is fully integrated into an industrial microelectronics process makes it an ideal candidate for national measurement institutes to realize and share a future quantum ampere.

Popular Summary

The ampere is the basic unit for presenting a measured electric current, whether in industry or in scientific research. The present international system of units (SI) still defines this unit based on measurement of the electromagnetic force between two current-carrying wires placed in parallel and at a distance from each other. But, this definition is no longer used in practice. Indeed, a fundamental shift toward electrical standards based on quantum effects already began several decades ago. Very accurate voltage and resistance standards have been established by using the Josephson effect of superconductors and the quantum Hall effect, respectively. A quantum standard of current, in other words, a standard for electron counting, accurate at about one part in ${10}^8$, will complete the quantum electrical standards. To achieve this standard requires an electron pump that is extremely accurate and has a high current yield in the nanoampere range. Relatively simple fabrication would also be highly advantageous to the broad utility of the standard. In this paper, we present the proof-of-principle demonstration of a new electron pump that promises to meet all these requirements.

An electron pump generates a quantized dc current when driven at a frequency $f$: $I=Nef$, where $e$ is the electron charge and $N$ is the number of electrons. A fundamental difficulty with quantized current sources comes from the necessity to have a very small system that can contain a well-defined number of charges. Such a requirement is at the limit of a typical academic nanofabrication facility. Our pump is, however, fabricated by exploiting the advanced silicon-on-insulator technology that has just entered commercial electronics. We use a silicon nanowire with two top gates in series, instead of the usual single-gate transistor geometry. A well-defined number of electrons is present on the central island between the two gates, which can regulate the electron flow at high speed under the control of an electric field. Robust and simple pumping at 650 MHz and 0.5 K is demonstrated. This frequency produces a current of the order of 1 nA, high enough to be used as a current standard. The relatively high temperature of operation, made possible by the very small size of the samples, will make it easier to build and operate such a standard.

The proof-of-principle demonstration of electron pumping in our device, along with its simple design and a fabrication process fully integrated in an industrial electronic technology, should make the device a strong candidate as the platform for national measurement institutes to realize and share a future quantum current standard.