Wireless technologies like Wi-Fi, Bluetooth, and 5G rely on radio frequency (RF) signals to send and receive data. A new prototype of an energy harvesting module – developed by a team led by scientists from the National University of Singapore (NUS) – can now convert ambient or ‘waste’ RF signals into DC voltage that can be used to power small electronic devices.
According to NUS, RF energy harvesting technologies can reduce battery dependency, extend device lifetimes, minimise environmental impact, and enhance the feasibility of wireless sensor networks and IoT devices in remote areas.
However, RF energy harvesting technologies face challenges due to low ambient RF signal power (typically less than -20dBm), where current rectifier technology either fails to operate or exhibits a low RF-to-DC conversion efficiency. Improving antenna efficiency and impedance matching can enhance performance, but this increases on-chip size.
To address these challenges, a team of NUS researchers, working in collaboration with scientists from Tohoku University (TU) in Japan and University of Messina (UNIME) in Italy, has developed a compact and sensitive rectifier technology that uses nanoscale spin-rectifier (SR) to convert ambient wireless radio frequency signals at power less than -20dBm to a DC voltage.
The team optimised SR devices and designed two configurations: a single SR-based rectenna operational between -62dBm and -20dBm, and an array of 10 SRs in series achieving 7.8 per cent efficiency and zero-bias sensitivity of approximately 34,500mV/mW. Integrating the SR-array into an energy harvesting module, they powered a commercial temperature sensor at -27dBm.
“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,” said Professor Yang Hyunsoo from the Department of Electrical and Computer Engineering at the NUS College of Design and Engineering.
“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.”
According to Prof Yang, the team optimised the spin-rectifiers to operate at low RF power levels available in the ambient, and integrated an array of such spin-rectifiers to an energy harvesting module for powering the LED and commercial sensor at RF power less than -20dBm.
“Our 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,” said Prof Yang.
The experimental research was carried out in collaboration with Professor Shunsuke Fukami and his team from TU, while the simulation was carried out by Professor Giovanni Finocchio from UNIME. The team’s findings are detailed in Nature Electronics.
The NUS research team is now exploring the integration of an on-chip antenna to improve the efficiency and compactness of SR technologies. The team is also developing series-parallel connections to tune impedance in large arrays of SRs, utilising on-chip interconnects to connect individual SRs.
The researchers also aim to collaborate with industry and academic partners for the advancement of self-sustained smart systems based on on-chip SR rectifiers.
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