Diagram of Power Generation
Overview diagram of power generation using ambient radio waves.

A highly sensitive rectifying element in the form of a nanowire backward diode can convert low-power 100 nanowatt microwaves into usable electricity. The newly developed diode has achieved a level of sensitivity more than 10 times higher than conventional Schottky barrier diodes.

Researchers at the Japan Science and Technology Agency (JST), Fujitsu Limited, and the Tokyo Metropolitan University expect this new technology 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, have difficulty converting low-power microwaves that are weaker than microwatts, which account for many of the ambient radio waves. There was a need, therefore, for a highly sensitive diode.

This research group succeeded in forming a backward diode that possesses excellent rectification characteristics, even within low voltage ranges, in a nanowire that has been miniaturized to a width of about one thousandth the width of a strand of hair.

The responsiveness (sensitivity) of a diode to microwaves largely depends on the steepness of the rectification characteristics and on diode size (capacity). Generally, Schottky barrier diodes, which utilize the rectification occurring at a 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, however, sensitivity to low-power microwaves weaker than microwatts was insufficient. This led to a demand for diodes with increased sensitivity.

The researchers shrunk the capacity of and miniaturized a backward diode that is capable of steep rectification operation with zero bias. 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 are formed by processing the thin film of a layered compound semiconductor into a disk shape via etching. Because the materials are prone to damage under processing, it is 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 nanocrystals with a diameter of 150nm comprised of n-type indium arsenide (n-InAs) and p-type gallium arsenide antimonide (p-GaAsSb) for the 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 an 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 sub-micron sized diode, which was difficult to do with conventional miniaturization process technology for compound semiconductors.

In testing the new technology at 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). 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).

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