Ambient RF signals could power electronic devices

Ubiquitous wireless technologies like Wi-Fi, Bluetooth and 5G rely on radio frequency (RF) signals to send and receive data. Now, a prototype energy-harvesting module is being used to convert ambient or ‘waste’ RF signals into direct-current (DC) voltage, which could power small electronic devices without the use of batteries.

RF energy-harvesting technologies will be essential going forward as they reduce battery dependency, extend device lifetimes, minimise environmental impact and enhance the feasibility of wireless sensor networks and IoT devices in remote areas where frequent battery replacement is impractical. However, RF energy-harvesting technologies face challenges due to low ambient RF signal power (typically less than -20 dBm), where current rectifier technology either fails to operate or exhibits a low RF-to-DC conversion efficiency. While improving antenna efficiency and impedance matching can enhance performance, this also increases on-chip size, presenting obstacles to integration and miniaturisation.

To address these challenges, researchers from the National University of Singapore (NUS), Tohoku University and the University of Messina developed a compact and sensitive rectifier technology that uses nanoscale spin rectifiers (SRs) to convert ambient wireless RF signals at power less than -20 dBm to a DC voltage. Their work has been described in the journal Nature Electronics.

“Harvesting ambient RF electromagnetic signals is crucial for advancing energy-efficient electronic devices and sensors; however, existing energy-harvesting modules face challenges operating at low ambient power due to limitations in existing rectifier technology,” explained NUS Professor Yang Hyunsoo, who spearheaded the project.

“For example, gigahertz Schottky diode technology has remained saturated for decades due to thermodynamic restrictions at low power, with recent efforts focused only on improving antenna efficiency and impedance-matching networks, at the expense of bigger on-chip footprints. Nanoscale spin-rectifiers, on the other hand, offer a compact technology for sensitive and efficient RF-to-DC conversion.”

The team optimised SR devices and designed two configurations: a single SR-based rectenna operational between -62 and -20 dBm; and an array of 10 SRs in series achieving 7.8% efficiency and zero-bias sensitivity of approximately 34,500 mV/mW. Integrating the SR array into an energy-harvesting module, they successfully powered a commercial temperature sensor at -27 dBm. These results “demonstrate that SR technology is easy to integrate and scalable, facilitating the development of large-scale SR arrays for various low-powered RF and communication applications”, according to Yang.

The researchers are now exploring the integration of an on-chip antenna to improve the efficiency and compactness of SR technologies. They are also developing series-parallel connections to tune impedance in large arrays of SRs, utilising on-chip interconnects to connect individual SRs. This approach aims to enhance the harvesting of RF power, potentially generating a significant rectified voltage of a few volts, thus eliminating the need for a DC-to-DC booster.

Ultimately, the researchers aim to collaborate with industry and academic partners for the advancement of self-sustained smart systems based on on-chip SR rectifiers. This could pave the way for compact on-chip technologies for wireless charging and signal detection systems.

“Despite extensive global research on rectifiers and energy-harvesting modules, fundamental constraints in rectifier technology remain unresolved for low ambient RF power operation,” said NUS’s Dr Raghav Sharma, first author on the paper.

“Spin-rectifier technology offers a promising alternative, surpassing current Schottky diode efficiency and sensitivity in low-power regime. This advancement benchmarks RF rectifier technologies at low power, paving the way for designing next-generation ambient RF energy harvesters and sensors based on spin rectifiers.”

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