Sunday, June 16, 2024

Nano Ampere Current Standard With 10-7 Uncertainty Using Silicon Quantum Dots

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The currents generated by two independent silicon quantum dots (single electron devices) matched with an uncertainty of about 4×10-7.

NTT Corporation (NTT) and the National Institute of Advanced Industrial Science and Technology (AIST), Japan have created a stable and reliable electric current using quantum dots. It is now possible to generate a current of 160 pico Amperes with a relative uncertainty of approximately 10-7.

Extremely precise measurement technology is crucial in fields like microfabrication, physics, and chemistry. To measure these currents, a ‘current standard’ is used based on Quantum Hall Resistance Standard and the Josephson Voltage Standard. The standard ensures such precise current measurement is realised by linking resistors and voltage sources in a way that leverages quantum mechanical phenomena through Ohm’s law. But,  such current standards’  relative uncertainty  increases as the current value decreases. For currents in nano Amperes and less, uncertainty below 10-3 could not be realised.

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To combat this, NTT and AIST focused on single-electron devices and precision current measurement technology, respectively, to make accurate measurements for small currents. With a finite number of electrons to flow through the conductor (single electron device), the electron flow and thus current can be controlled and measured precisely. The currents generated by two independent silicon quantum dots (single electron devices) matched with an uncertainty of about 4×10-7. Also, by adding the two currents together, they successfully doubled the current while maintaining a small uncertainty. 

This precise current generation technology and current comparison technology will serve as the ‘standard’ for the measurement of minute currents below nano-ampere, and contribute to the improvement of current measurement accuracy in semiconductor microfabrication, chemical measurement and radiation measurement.

In the research two tiny silicon quantum dots (Element A and Element B), each a few tens of nanometers in size, were created using advanced microfabrication technology. To generate a current, a negative voltage is initially applied to two gate electrodes, creating quantum dots in a silicon wire. Then, by applying a positive voltage to one of the gate electrodes, the energy barrier in the silicon wire is lowered, guiding electrons into the quantum dot. By adjusting voltages, a series of operations continuously transfers electrons one by one, generating a current. The actual results, shown in the experiment, reveal a region (current plateau) where the current remains constant despite changes in voltage.

By successfully harnessing silicon quantum dots to create a stable and reliable electric current. The researchers addressed challenges in precise measurement technologies, particularly for extremely small currents. The achievement not only ensures consistency in the fundamental rules of the microscopic world but also holds the potential to power multiple devices simultaneously, paving the way for advancements in current comparison and multiplication techniques.


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