Ubiquitous wireless technologies like Wi-Fi, Bluetooth, and 5G rely on radio frequency (RF) signals to send and receive data. A new prototype energy harvesting module, developed by a team led by scientists at the National University of Singapore (NUS), can now convert ambient or “residual” RF signals into direct current (DC) voltage. This can be used to power small electronic devices without using batteries.
RF energy harvesting technologies, such as this one, are essential because they reduce battery dependency, extend device life, minimize environmental impact, and improve 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 below -20 dBm), where current rectification technology either does not work or has low RF-DC conversion efficiency. While improving antenna efficiency and impedance matching can improve performance, it also increases the chip size, which is a barrier to integration and miniaturization.
To address these challenges, a team of NUS researchers, working 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 spin rectifier (SR) to convert ambient wireless radio frequency signals at a power below -20 dBm into a DC voltage.
The team optimized the SR devices and designed two configurations: 1) a single SR-based rectenna operating between -62 dBm and -20 dBm, and 2) an array of 10 SRs in series achieving an efficiency of 7.8% and a zero-bias sensitivity of about 34,500 mV/mW. By integrating the SR array into an energy harvesting module, they successfully powered a commercial temperature sensor at -27 dBm.
“Harvesting ambient RF electromagnetic signals is essential for advancing energy-efficient electronic devices and sensors. However, existing energy harvesting modules face challenges when operating at low ambient power due to the limitations of existing rectification technology,” explained Professor Yang Hyunsoo from the University of California, Berkeley. Department of Electrical and Computer Engineering At NUS College of Design and Engineeringwho led the project.
Professor Yang added: “For example, gigahertz Schottky diode technology has remained saturated for decades due to low-power thermodynamic restrictions, with recent efforts focusing only on improving antenna efficiency and impedance matching networks, at the expense of larger chip footprints. Nanoscale spin rectifiers, on the other hand, offer a compact technology for sensitive and efficient RF-to-DC conversion.”
Discussing the team’s breakthrough technology, Professor Yang said, “We optimized the spin rectifiers to operate at low RF power levels available in the environment, and integrated a set of these spin rectifiers with an energy harvesting module to power the LED and commercial sensor at RF power below -20 dBm. Our results demonstrate that SR technology is easy to integrate and scalable, facilitating the development of large-scale SR networks for various low-power RF and communication applications.”
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 results were published in the prestigious journal, Natural electronicsJuly 24, 2024.
Spin rectifier technology for low power operation
Advanced rectifiers (Schottky diodes, tunnel diodes and two-dimensional MoS2), have achieved yields of 40 to 70% at PRF ≥ -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 regimes (PRF
Nanoscale spin rectifiers can convert RF signal into DC voltage by using the spin diode effect. Although SR-based technology has surpassed the sensitivity of Schottky diode, the low-power efficiency remains low (
To improve efficiency and achieve on-chip operation, the SRs were coupled in an array arrangement, with the small coplanar waveguides of the SRs being used to couple the RF power, resulting in a compact on-chip area and high efficiency. A key finding is that the well-known VCMA-driven self-parametric effect in magnetic tunnel junction-based spin rectifiers contributes significantly to the low-power operation of the SR arrays, while also improving their bandwidth and rectification voltage. In a comprehensive comparison with Schottky diode technology in the same ambient situation and from an evaluation of previous literature, the research team found that SR technology could be the most compact, efficient, and sensitive rectifier technology.
Commenting on the significance of their findings, Dr Raghav Sharma, first author of the paper, said, “Despite extensive global research on rectifiers and energy harvesters, fundamental constraints of rectifier technology remain unsolved for low ambient RF power operation. Spin rectifier technology offers a promising alternative, surpassing the efficiency and sensitivity of current Schottky diodes in the low power regime. This advancement enables benchmarking of low power RF rectifier technologies, paving the way for designing next-generation ambient RF energy harvesters and sensors based on spin rectifiers.”
Next steps
The NUS research team is currently investigating on-chip antenna integration to improve the efficiency and compactness of SR technologies. The team is also developing series-parallel connections to tune impedance in 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, eliminating the need for a DC-to-DC amplifier.
The researchers also want to collaborate with industrial and academic partners to advance autonomous smart systems based on integrated SR rectifiers. This could pave the way for compact integrated technologies for wireless charging and signal detection systems.