Highly sensitive diode converts microwaves to electricity

The Japan Science and Technology Agency (JST), Fujitsu and the Tokyo Metropolitan University have developed a highly sensitive rectifying element in the form of a nanowire backward diode, which can convert low-power microwaves into electricity. 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.

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 as means for creating sensor networks that function without batteries. One such example re-uses 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.

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 utilise 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 (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.

The researchers carried out development to create a diode with higher sensitivity. Specifically, they shrank 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 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 150 nm comprising 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 ends 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 submicron-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 10 times the sensitivity of conventional Schottky barrier diodes — said to be a world first.

In testing the new technology in the microwave frequency of 2.4 GHz, which is currently used in the 4G LTE and Wi-Fi communication line standards for mobile phones, the sensitivity was 700 kV/W — roughly 11 times that of the conventional Schottky barrier diode (with a sensitivity of 60 kV/W). Therefore, the technology can efficiently convert 100 nW-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.

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, 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.

Image credit: ©stock.adobe.com/au/silvae

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