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Net Journal 1

An ultimate digital device that uses only one electron for on-off control

Fukui Takashi, Doctor of Engineering

Dr. Fukui: A single-electron transistor produces an on or off status by creating conductor islands (dots) of approximately 10 nanometers in diameter on a very small semiconductor base and charging/discharging one electron to and from each dot. Since it uses the Coulomb blockade phenomenon (in which the potential of a dot is increased by charging an electron and stopping other electrons from entering), it is also known as an ultimate energy-saving transistor (Ex. 1).

While the theory of the single-electron transistor itself had already been presented at the end of the 1980s, it was considered unfeasible with the technology of the period, since processing to reduce the dot size to approximately 10 nanometers was necessary for operation at room temperature. However, the advance of microfabrication technology in the 1990s enabled the production of a variety of nanostructures, and attempts to realize single-electron transistors began in many countries.

Only one single-electron transistor was created at first as a product of coincidence, and was not usable as a circuit element. Transistors for use as integrated circuits must be produced not only to demonstrate the physical theory, but also to seek a way for practical application. Our aim was to develop single-electron transistors that can be integrated to function as a logic circuit. While many researchers worked on single-electron transistors made of silicon (since most common semiconductor devices are made of silicon), our laboratory uses compound semiconductors consisting of chemical compounds with gallium, indium and other metals.

---- You have been engaged in research on compound semiconductors for over 30 years since the development of a semiconductor laser at the NTT Laboratory. What are the advantages of a single-electron transistor using compound semiconductors?

Dr. Fukui: To integrate transistors, it is necessary to precisely design their positional relationship. However, microfabrication on a scale of several tens of nanometers is not possible with the conventional top-down system. What can be useful here is a bottom-up system to form a nanostructure through self-organized growth of atomic and molecular crystals contrary to the top-down system.

Compound semiconductors are suited for formation of a bottom-up-type microscopic circuit network, and their fabrication technology is mature. We have realized a nanostructure with a dot diameter and conductor width of several tens of nanometers. It has a well-designed layout using a technology called organometallic vapor phase selective epitaxy (Ex. 2), which combines the top-down and bottom-up systems. We succeeded in confirming the operation of an AND/NANO logic circuit by integrating four single-electron transistors in an experiment in 2003. In 2005, we were also successful in producing an AND/XOR one-bit adder that works with three transistors.

---- The results of these experiments were presented in Applied Physics Letters, the American Physical Society's journal of academic papers, with the cover featuring an electron micrograph of a logic circuit formed by integrating single-electron transistors (Photo 1). I think this is a world-class research achievement that realizes a truly ultimate digital device.



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