JST, Fujitsu, and Tokyo Metropolitan University Develop Highly Sensitive Diode, Converts Microwaves to Electricity

Estimated reading time: 9 minutes

Research Details

The researchers carried out development to create a diode with higher sensitivity (figure 2). Specifically, they shrunk the capacity of and miniaturized a backward diode that is capable of steep rectification operations with zero bias (4), as rectification occurs by joining two different types of semiconductors and current flows with a different principle (tunnel effect) than conventional Schottky barrier diodes.

Conventional backward diodes were formed by processing the thin film of a layered compound semiconductor into a disk shape via etching. Nonetheless, because the materials are prone to damage under processing, it was difficult to finely process diodes to a submicron size and operate them.

By adjusting the ratio (composition) of the constituent elements of the connected semiconductor materials and, at a minute level, the density of the added impurities, the researchers succeeded in growing crystals in nanocrystals with a diameter of 150nm comprised of n-type indium arsenide (n-InAs) and p-type gallium arsenide antimonide (p-GaAsSb) for a tunnel junction structure necessary for the characteristics of the backward diode. Moreover, in the process for implanting insulating material around the nanowire and the process for forming electrode film with metal on both end of the wire, a new technology was used for mounting that does not damage the nanowire. As a result, they were able to form a sub-micron sized diode, which was difficult to do with conventional miniaturization process technology for compound semiconductors. , and thereby succeeded, for the first time in the world, in developing a nanowire backward diode with over 10 times the sensitivity of conventional Schottky barrier diodes (figure 3).

In testing the new technology in the microwave frequency of 2.4GHz, which is currently used in the 4G LTE and Wi-Fi communication line standards for mobile phones, the sensitivity was 700kV/W, roughly 11 times that of the conventional Schottky barrier diode (with a sensitivity of 60KV/W) (figure 4). Therefore, the technology can efficiently convert 100nW-class low-power radio waves into electricity, enabling the conversion of microwaves emitted into the environment from mobile phone base stations in an area that is over 10 times greater than was previously possible (corresponding to 10% of the area in which mobile phone communications are possible). This has led to expectations that it can be used as a source of power for sensors (figure 5).

Future Plans

In the future, it is expected that the newly-developed nanowire backward diode will be applied in using plentiful ambient radio wave energy in 5G communications, serving as a stable power source of sensors and contributing to battery-free sensors used to monitor infrastructure such as constructions and buildings.

Going forward, the research group will further increase the sensitivity of the diode, optimize the diode-integrated antenna, and add power control for voltage consistency, aiming to realize a technology that can generate power anywhere using ambient radio waves.

Diagrams

A small-scale antenna receives radio waves (microwaves) that are emitted in the surrounding environment, such as from mobile phone base stations, and uses them as an energy source to power sensors. As the electricity conversion element, highly sensitive diodes are required to enable low-power radio waves to be rectified.

Unlike conventional Schottky barrier diodes, backward diodes have steeply-sloped rectifying characteristics with zero bias.

Just by growing crystals, without using an etching process, extremely thin nanowires with a width of about one thousandth that of a strand of hair, were formed on a semiconductor substrate. With this process, electrodes can be formed without infecting damage to the nanowire.

In the microwave frequency band of 2.4GHz, which is currently used for 4G LTE and Wi-Fi, a level of sensitivity was achieved that is 11 times that of a conventional Schottky barrier diode.

By using the newly-developed nanowire backward diode as the rectifying element, electricity can be converted in an area that is over 10 times as large as that achieved using a conventional Schottky barrier diode (corresponding to 10% of the area in which mobile phone communications are possible).

In the microwave frequency band of 2.4GHz, which is currently used for 4G LTE and Wi-Fi, a level of sensitivity was achieved that is 11 times that of a conventional Schottky barrier diode.

Title of Journal Article

"Highly Sensitive p-GaAsSb/n-InAs Nanowire Backward Diodes for Low-Power Microwaves"

Supplementary Note

The nanowire backward diode was prototyped at the Device & Materials Research Center (in Atsugi, Kanagawa prefecture) of Fujitsu Laboratories Ltd.

(1) Backward diode
In contrast to conventional Schottky barrier diodes, these diodes operate by using the phenomenon of tunneling. They enable excellent rectification operations even within small voltage ranges in which conventional diodes are unable to achieve sufficient rectification.
(2) Nanowire
Wire-shaped semiconductors so thin that their width is measured in nanometers (nm). Rather than through top-down processing such as etching, they can be built bottom up through crystal formation.
(3) Schottky barrier diode
Diodes that use the energy known as a Schottky barrier, which is produced through a junction of a semiconductor and metal, for rectification.
(4) Zero bias
A condition in which there is zero voltage. With the harvesting of energy from environmental radio waves, zero bias operations are needed because power cannot be consumed for the adjustment of the operating voltage.

About Japan Science and Technology Agency (JST)

JST is an organization that leads Japan’s science and technology (S&T) development as an innovation navigator. We aim to contribute to the lives of people and the achievement of a sustainable society by promoting S&T for the purpose of opening up opportunities in innovation. Since its foundation, JST’s many outstanding achievements accomplished in collaboration with the government, universities, the industrial sector and public have been earned global recognition. www.jst.go.jp/EN/index.html

About Fujitsu

Fujitsu is the leading Japanese information and communication technology (ICT) company, offering a full range of technology products, solutions, and services. Approximately 132,000 Fujitsu people support customers in more than 100 countries. We use our experience and the power of ICT to shape the future of society with our customers. Fujitsu Limited (TSE: 6702) reported consolidated revenues of 4.0 trillion yen (US $36 billion) for the fiscal year ended March 31, 2019. For more information, please see www.fujitsu.com.

About Tokyo Metropolitan University

Tokyo Metropolitan University (TMU) is the only university operated by the Tokyo Metropolitan Government. By providing an education that capitalizes on TMU’s strong points, TMU will produce personnel who can operate in response to societal changes, will promote a variety of basic and applied research as well as conduct research on urban issues. This will help Tokyo to advance and in turn provide future insight for the rest of the world.

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