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

Estimated reading time: 9 minutes

Key Points

• In entering the true IoT era, there is a demand for highly sensitive diodes that can convert low-power radio waves (microwaves) in the surrounding environment into electricity.
• For the first time in the world, the newly-developed nanowire backward diode achieved more than 10 times the sensitivity of conventional Schottky barrier diodes.
• Optimized design of the diode and integrating antenna, and addition of power controls, are driving expectations for battery-free sensors powered by ambient radio waves.

The Japan Science and Technology Agency (JST), Fujitsu Limited, and the Tokyo Metropolitan University today announced that they developed a highly sensitive rectifying element in the form of a nanowire backward diode, which can covert low-power microwaves into electricity. Through JST's Strategic Basic Research Programs, the technology was developed by researchers led by Kenichi Kawaguchi of Fujitsu Limited and Professor Michihiko Suhara of the Tokyo Metropolitan University. The new technology is expected to play a role in harvesting energy from radio waves in the environment, in which electricity is generated from ambient radio waves, such as those emitted from mobile phone base stations.

To facilitate the commencement of a true IoT era, energy harvesting from environmental radio waves is receiving attention as a means for building sensor networks that do not require batteries. Conventional rectifying elements, however, due to their low-voltage rectification characteristics and element sizes, had difficulties in converting low-power microwaves that are weaker than microwatts (μW), which account for many of the ambient radio waves, to electricity. There was a need, therefore, for a highly sensitive diode.

This research group succeeded in forming a backward diode(1) that possesses excellent rectification characteristics even within low voltage ranges in a nanowire(2) that has been miniaturized to a width of about one thousandth the width of a strand of hair. The newly-developed nanowire backward diode achieved a level of sensitivity more than 10 times higher than conventional Schottky barrier diodes.(3)

With this technology, microwaves with a power level of 100 nanowatts (nW) can be converted to electricity. Going forward, as the research group optimize the design of the diode and the radio wave-collecting antenna while adding power control for constant voltage, there are high expectations for the realization of energy harvesting from environmental radio waves.

The results of this research will be announced on September 26, 2019, at the European Solid-State Device Research Conference (ESSDERC), an international conference being held in Krakow, Poland.

• These results were generated under the following program, research fields, and research area.
• Team-oriented research (CREST), Strategic Basic Research Programs
• Research field: "Scientific Innovation for Energy Harvesting Technology using Minute Sources of Energy"
• (Research Supervisor: Kenji Taniguchi, Professor Emeritus, Osaka University
• Deputy Research Supervisor: Hiroyuki Akinaga, Principal Research Manager, Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology(AIST))
• Title of research area: "Research and development of ambient RF energy-harvesting devices using semiconductor nanowires"
• Research director: Kenichi Kawaguchi (Deputy General Manager, Network Product Business Unit, Wireless System Dept., Fujitsu Limited)
• Research period: October 2016 to March 2020

In this field, JST aims to create innovative basic technologies that convert the unused and minute energy from heat, light, vibrations, radio waves, and living organisms existent in various environments into electricity (energy harvesting) for the purpose of use in sensors, information processing devices, and other devices. In the above research topic, a highly sensitive backward diode using tunneling current as the operating principle was made smaller in capacity, through submicron-sized, minute semiconductor nanowires. This enables the creation of receiving devices with dramatically improved sensitivity. Also, by embedding optimized power conversion circuitry into the nanowire backward diode, the research group will conduct proof of principle testing on the power conversion of low-power ambient radio waves.

Research Background and Circumstances

In preparation for the commencement of the true IoT era, energy harvesting technologies, which transform the minute sources of energy in the surrounding environment into electricity, have come under the spotlight in recent years as means for creating sensor networks that function without batteries. One such example reuses as electricity the low-power radio waves (microwaves), ubiquitous in open space, that are emitted from mobile phone base stations, for use in communications. Equipment used in generating electricity from ambient radio waves consists of a radio wave power generating element, which includes an antenna for collecting radio waves and a rectifying element (diode) that rectifies the radio waves (figure 1).

The responsiveness (sensitivity) of a diode to microwaves largely depends on the steepness of rectification characteristics and on diode size (capacity). Generally, Schottky barrier diodes, which utilize the rectification occurring at the junction formed between a metal and a semiconductor, are used as the diodes for power conversion. Due to rectification characteristics becoming slow at extremely low voltages and the size of elements being larger than several micrometers (μm), however, sensitivity to low-power microwaves weaker than microwatts (μW) was insufficient, and it was difficult to convert ambient radio waves into electricity. This led to a demand for diodes with increased sensitivity.

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