An Experiment in Progress
"This could become a fundamental technology enabling us to use a mobile phone for a year without recharging the battery. The development of low-power electronic devices is essential for realizing the ubiquitous society," says Hiroshi Inokawa of NTT Basic Research Laboratories. Inokawa is a leading researcher of the Nanodevices Research Group of the labs' Physical Science Laboratory. A group consisting of Inokawa, Katsuhiko Nishiguchi, and other members of the Nanodevices Research Group has successfully developed "single-electron memory," which reduces the total number of electrons stored in a memory area to 1/10,000th that of conventional DRAM (dynamic random-access memory). This revolutionary innovation greatly reduces the amount of power it takes to store information.
Hiroshi Inokawa
Nanodevices Research Group Physical Science Laboratory
Before describing what single-electron memory is, we should first explain what a "single-electron device" is. Transistors perform a wide range of actions through the flow of electrons between two electrodes, called the "source" and the "drain." Ordinary transistors have around 100,000 electrons in the channel linking the source and drain electrodes, and accordingly consume a large amount of power. Conversely, a single-electron device has only one electron (or a countable number of electrons).
Since only one electron is being handled, power consumption is extremely low, and the circuit size is in the order of nanometer (one billionth of a meter); this minute construction allows you to make even large-scale integrated circuits very compact. In other words, single-electron devices perform a lot of work using a small amount of energy.
In the past, such devices have only been able to operate at ultra-low temperatures of-273oC, but in 1994, NTT succeeded in developing the world's first single-electron transistor capable of operating at room temperature.
Inokawa and his colleagues have expanded on this technology, researching single-electron memory as a new principle for high-capacity information storage, by manipulating one electron at a time. In the spring of 2004, Inokawa's team announced that it had successfully increased the storage capacity five-fold over conventional memory devices.
The basic component consists of two "gates" and a single-electron "dot." When voltages are applied alternatingly, the two gates open and close, sending one electron at a time into the dots, which is the memory area. The number of electrons stored can be detected by measuring the current flowing through the single-electron transistors located close to the dots. In other words, if there is an electron in the dot, the repulsion against the electrons in the measuring transistor become intense, impeding the flow of current.
The team increased the total number of electrons stored in the memory area, confirming that it could store a maximum of 50 electrons, and accurately control up to 40. A single-electron device represents one bit of information, expressed by the presence of 0 or 1 electron. The new device storing 40 electrons can handle information corresponding to more than 5 bits.
The width of the wiring in the basic component is just 35 nanometers. "The thinner the wiring," explains Inokawa, "the more you can control the fluctuations in the number of electrons manipulated, enabling more stable operation." Although the single-electron memory would only operate at low temperatures as of the spring of 2004, by winter of that year, they succeeded at operating it at room temperature.
Katsuhiko Nishiguchi Holds Nanometer-size Electronic Circuits
A truly vast number of fast-operating semiconductors are used in the world of telecommunications equipment. As the speed at which information is communicated increases, the amount of energy consumed is soaring. As of 1990, the NTT Group's energy consumption was about 3,450 million kWh; as of fiscal 2004, it is about 7,740 million kWh. "Our levels of energy consumption keep increasing," says Inokawa; "we absolutely need a new technology to reduce them. One such new technology being researched is single-electron devices and single-electron memory."
Inokawa's team has been researching single-electron devices for 10 years. The single-electron memory will not start appearing in our PCs that we use until the year 2012 to 2018; but this technology holds much promise. In the "The Nihon Keizai Shimbun's survey of technological achievements in Japan 2004" published by The NIKKEI WEEKLY in January 2005, the new single-electron device ranked 5th.
"Reduction of environmental burdens is not the only goal of the research into single-electron memory," says Inokawa. "But it can be operated at low power consumption, and the higher density makes it possible to provide more functionality using fewer raw materials. So as a result, leading-edge technology is contributing to the environment."
The team is currently studying the possibilities of operating a large-scale circuit from various viewpoints. However, "We actually have not fully analyzed why we can accurately operate the single-electron memory at room temperature beyond expectations," says Nishiguchi. "We are continuing to work on this analysis," he adds with a smile.
Unexpected results appear from an experiment: the wondrous aspect of science is also leading to reductions in environmental burdens.
