Widespread wireless technologies such as 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 direct current (DC) voltage ). This can be used to power small electronic devices without the use of batteries.
RF energy harvesting technologies such as this are essential as they reduce battery dependency, extend device lifetime, minimize environmental impact, and improve the feasibility of wireless sensor networks and IoT devices in remote areas where frequent replacement of battery 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 does not work or exhibits low RF-conversion efficiency. in-DC. While improving antenna efficiency and impedance matching can improve performance, it also increases chip size, presenting barriers 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 the nanoscale rectifier (SR). ) to convert ambient wireless radio frequency signals with power less than -20 dBm into a DC voltage.
The team optimized the SR devices and designed two configurations: 1) a single SR-based rectifier operating between -62 dBm and -20 dBm, and 2) an array of 10 SRs in series achieving 7.8% efficiency and bias sensitivity zero of approximately 34,500 mV/mW. By integrating the SR array into an energy harvesting module, they successfully powered a commercial temperature sensor at -27 dBm.
“Receiving ambient RF electromagnetic signals is essential to the advancement of 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 Professor Yang Hyunsoo from the Department of Electrical and Computer Engineering in the NUS College of Design and Engineering, who led the project.
Prof Yang added, “For example, gigahertz Schottky diode technology has remained saturated for decades due to low-power thermodynamic limitations, with recent efforts focused only on improving antenna efficiency and impedance matching networks, in on the back of larger on-chip footprints. On the other hand, nanoscale spin rectifiers provide a compact technology for sensitive and efficient RF-to-DC conversion.”
Elaborating on the team’s breakthrough technology, Prof Yang said: “We optimized the spin rectifiers to operate at low levels of ambient RF power and integrated an array of such spin rectifiers into an energy harvesting module to power the LEDs and commercial sensor with RF power less than -20 dBm Our results show that the SR technology is easy to integrate and scalable, facilitating the development of large-scale SR arrays for various low-power RF 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, Nature Electronics, on July 24, 2024.
Spin rectifier based technology for low power operation
State-of-the-art rectifiers (Schottky diodes, tunnel diodes and two-dimensional MoS2), have achieved 40-70% efficiency in 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 the RF signal to a DC voltage using the spin diode effect. Although the SR-based technology surpassed the sensitivity of the Schottky diode, the low-power efficiency is still low (
To improve manufacturing and achieve on-chip operation, the SRs were combined in an array arrangement, with small co-planar wavelengths in the SRs used for RF power coupling, resulting in a compact on-chip area and efficiency of up. One of the key findings is that the self-parametric effect induced by the well-known VCMA in spin rectifiers based on magnetic tunnel junctions contributes significantly to the low-power operation of SR arrays, also increasing 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 SR technology can be the most compact, efficient and sensitive rectifier technology.
Commenting on the significance of their results, Dr Raghav Sharma, first author of the paper, shared: “Despite extensive global research on rectifiers and energy harvesting modules, fundamental limitations in rectifier technology remain unresolved for low-power operation of ambient RF. Spin-rectifier technology offers a promising alternative, surpassing the efficiency and sensitivity of the current Schottky diode in the low-power regime This advance marks low-power RF rectifier technologies, paving the way for the design of the next generation of rectifier-based ambient RF energy harvesters and sensors.
Next steps
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 resistance in large SR arrays, using on-chip interconnects to connect individual SRs. This approach aims to increase RF power gain, potentially generating a significant rectified voltage of several volts, thus eliminating the need for a DC-to-DC amplifier.
The researchers also aim to collaborate with industry and academic partners for the advancement of self-sustaining 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.