Widespread wireless technologies such as Wi-Fi, Bluetooth and 5G rely on radio frequency (RF) signals to send and receive data. A new energy harvesting prototype – developed by a team led by scientists from the National University of Singapore (NUS) – can now convert ambient or “waste” RF signals into direct current (DC) voltage. This could be used to power small electronic devices without using batteries.
RF energy harvesting technologies, such as these, are essential because they reduce battery dependence, extend device life, reduce environmental impact, and enhance the viability of wireless sensor networks and IoT devices in remote areas where frequent battery replacement is impractical.
However, RF energy harvesting techniques face challenges due to the low ambient RF signal power (typically less than -20 dBm), where the rectifier technology fails to work or exhibits low RF-to-DC conversion efficiency. While improving antenna efficiency and impedance matching can enhance performance, this also increases the chip size, which poses obstacles to integration and miniaturization.
To address these challenges, a team of researchers at the National University of Singapore, in collaboration with scientists from Tohoku University (TU) in Japan and the University of Messina (UNIME) in Italy, has developed a compact and sensitive rectifier technology that uses a nanoscale circulating rectifier (SR) to convert ambient RF signals with a power of less than -20 dBm into DC voltage.
The team optimized the SR devices and designed two configurations: 1) a single SR-based straight antenna operating between -62 dBi and -20 dBi, and 2) an array of 10 SR devices in series achieving 7.8% efficiency and a zero bias sensitivity of about 34,500 mV/mW. By integrating the SR array into an energy harvesting module, they successfully operated a commercial temperature sensor at -27 dBi.
“Harvesting ambient electromagnetic signals is crucial for the development of energy-efficient electronic devices and sensors. However, current energy harvesting units face challenges in operating at low ambient power due to limitations in current rectifier technology,” explained Professor Yang Hyun Su of MIT. Department of Electrical and Computer Engineering In the Faculty of Design and Engineering, National University of Singaporewho led the project.
“For example, gigahertz Schottky diodes have been saturated for decades due to thermodynamic limitations at low power, with recent efforts focusing only on improving antenna efficiency and impedance matching networks, at the expense of larger on-chip footprints,” Professor Yang added. “Nano-rotating rectifiers, on the other hand, offer a compact technology for sensitive and efficient RF-to-DC conversion.”
Explaining the team’s groundbreaking technology, Professor Yang said: “We optimized the gyro-rectifiers to operate at low RF power levels available in the ambient environment, and integrated a set of these gyro-rectifiers into an energy harvesting module to drive a commercial LED and sensor at RF power below -20 dBm. Our results demonstrate that the gyro-rectifier technology is easy to integrate and scalable, facilitating the development of large-scale gyro-rectifier sets for various RF and low-power communications applications.”
The experimental research was conducted in collaboration with Professor Shunsuke Fukami and his team from the University of Tokyo, while the simulation was conducted by Professor Giovanni Finocchio from the University of Tokyo. The results were published in the prestigious journal, Nature ElectronicsOn July 24, 2024.
Rectifier-based technology for low-power operation
Advanced components (Schottky diodes, tunnel diodes, 2D MoS diodes)2), and its efficiency reached 40-70% at PRadio Frequencies ≥ -10 dBm. However, the ambient RF power available from RF sources such as Wi-Fi routers is less than -20 dBm. Development of high-efficiency rectifiers for low-power systems (PRadio Frequencies
Nano-rotary rectifiers can convert RF signal into DC voltage using the rotary diode effect. Although the SR technology has surpassed the sensitivity of Schottky diode, the low power efficiency is still low (
To improve the output and achieve on-chip operation, the SRs were connected in an array arrangement, with small common-plane waveguides on the SRs used to couple the RF power, resulting in compact on-chip area and high efficiency. A key finding is that the well-known VCMA-driven self-modulating effect in magnetic tunnel junction-based spin rectifiers contributes significantly to the low-power operation of the SR arrays, while enhancing the bandwidth and rectification voltage. In a comprehensive comparison with Schottky diode technology in the same ambient situation and from previous literature evaluation, the research team found that the SR technology may be the most compact, efficient and sensitive rectifier technology.
Commenting on the significance of their findings, Dr. Raghav Sharma, the first author of the paper, said: “Despite extensive global research on rectifiers and energy harvesters, fundamental limitations in rectifier technology remain unresolved for low-ambient power RF operation. Rotary rectifier technology offers a promising alternative, surpassing the efficiency and sensitivity of the current Schottky diode in the low-power regime. This advance compares RF rectifier technologies at low power, paving the way for the design of next-generation RF ambient energy harvesters and sensors based on rotary rectifiers.”
Next steps
The NUS research team is now exploring the possibility of integrating an on-chip antenna to improve the efficiency and compactness of SR technologies. The team is also developing series and parallel connections to tune the impedance of large SR arrays, using on-chip interconnects to connect individual SRs. This approach aims to improve RF energy harvesting, potentially generating a large rectified voltage of a few volts, thereby eliminating the need for a DC-to-DC booster.
The researchers also aim to collaborate with industry and academic partners to develop self-sustainable smart systems based on on-chip SR components. This could pave the way for on-chip technologies for wireless charging and signal detection systems.
