Researchers power IoT devices from ambient radio waves
October 2, 2019
Japanese researchers have developed a way to power IoT devices from ambient radio waves in the atmosphere. The joint research was carried out by the Japan Science & Technology Agency (JST), Fujitsu and Tokyo Metropolitan University.
In the IoT era, there is a demand for highly sensitive diodes that can convert low-power radio waves (microwaves) in the surrounding environment into electricity. The researchers have developed a nanowire backward diode that achieved more than ten times the sensitivity of conventional Schottky barrier diodes.
By optimising the design of the diode and integrating antenna, and adding power controls, they hope this can achieve battery-free sensors powered by ambient radio waves.
The highly sensitive rectifying element has been developed in the form of a nanowire backward diode, which can convert low-power microwaves into electricity. Through JST's strategic basic research programmes, the technology was developed by researchers led by Kenichi Kawaguchi of Fujitsu and Michihiko Suhara of Tokyo Metropolitan University. The 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.
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, 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 that possesses rectification characteristics even within low voltage ranges in a nanowire that has been shrunk 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 ten times higher than conventional Schottky barrier diodes.
With this technology, microwaves with a power level of 100nW can be converted to electricity. Going forward, as the research group optimise the design of the diode and the radio wave-collecting antenna while adding power control for constant voltage, there are expectations for the realisation of energy harvesting from environmental radio waves.
The results of this research were announced last week at the European Solid-State Device Research Conference, an international conference held in Krakow, Poland.
Using this technology, JST aims to create basic technologies that convert the unused and minute energy from heat, light, vibrations, radio waves and living organisms existent in various environments into electricity for use in sensors, information processing devices and other devices.
In the research, a highly sensitive backward diode using tunnelling current as the operating principle was made smaller in capacity, through submicron-sized, minute semiconductor nanowires. This enables the creation of receiving devices with improved sensitivity. Also, by embedding optimised power conversion circuitry into the nanowire backward diode, the research group plans to conduct proof of principle testing on the power conversion of low-power ambient radio waves.
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 ubiquitous in open space that are emitted from mobile phone base stations. 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.
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 use 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 micrometres, however, sensitivity to low-power microwaves weaker than microwatts was insufficient, and it was difficult to convert ambient radio waves into electricity. This led to a demand for diodes with increased sensitivity.
The researchers thus created a diode with higher sensitivity. Specifically, they shrunk the capacity of and miniaturised a backward diode that is capable of steep rectification operations with zero bias, 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 process diodes finely 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 miniaturisation process technology for compound semiconductors, and thereby succeeded in developing a nanowire backward diode with over ten times the sensitivity of conventional Schottky barrier diodes, as shown in the diagram.
In testing the technology at the microwave frequency of 2.4GHz, which is used in 4G LTE and wifi communications for mobile phones, the sensitivity was 700kV/W, roughly 11 times that of the conventional Schottky barrier diode with a sensitivity of 60kV/W. 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 ten 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.
In the future, it is expected that the 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 plan to increase the sensitivity of the diode, optimise the diode-integrated antenna, and add power control for voltage consistency, aiming to realise a technology that can generate power anywhere using ambient radio waves.