Simon Hemour

Towards Low-Power High-Efficiency RF and Microwave Energy Harvesting

Since the very beginning of RF and microwave integrated techniques and energy harvesting, Schottky diodes are most often used in mixing and rectifying circuits. However, in specific μW power-harvesting applications, the Schottky diode technique fails to provide a satisfactory RF-dc conversion efficiency mainly because of its high zero-bias junction resistance. This paper examines the state-of-the-art low-power microwave-to-dc energy conversion techniques. A comprehensive picture of the state-of-the-art on this aspect is given graphically, which compares different technologies such as transistor, diode, and CMOS schemes. Subsequent to the highlighted limitations of current devices, this work introduces, for the first time, a nonlinear component for low-power rectification based on a recent discovery in spintronics, namely, the spindiode. Along with an analysis of the role of nonlinearity and zero bias resistance in the rectification process of the spindiode, it is shown how the spindiode could enhance the rectification efficiency even at a very low-power level and how this technique would shift the design paradigms of diode-based devices and circuits.

Radio-Frequency Rectifier for Electromagnetic Energy Harvesting: Development Path and Future Outlook

The roadmap evolution and historical milestones of electromagnetic energy conversion techniques and related breakthroughs over the years are reviewed and presented with particular emphasis on low-density energy-harvest technologies. Electromagnetic sources responsible for the presence of ambient radio-frequency (RF) energy are examined and discussed. The effective use and recycling of such an ambient electromagnetic energy are the most relevant and critical issue for the current and future practicability of wireless energyharvesting devices and systems. In this paper, a set of performance criteria and development considerations, required to meet the need of applications of ambient electromagnetic energy harvesting, are also derived from the radiating source analysis. The criteria can be calculated from a simple measurement of the I-V nonlinear behavior of RF rectification devices such as diodes and transistors, as well as linear frequency behavior (S-parameters). The existing rectifying devices are then reviewed in light of the defined performance criteria. Finally, a technological outlook of the performances that can be expected from different device technologies is assessed and discussed. Since the proposed spindiode technology would present the most promising device platform in the development of the most useful ambient energy harvesters, a special highlight of this disruptive scheme is provided in the presentation of this work.

Microwave reflection imaging using a magnetic tunnel junction based spintronic microwave sensor

A far-field microwave imaging technique has been developed using a spintronic sensor based on a magnetic tunnel junction (MTJ). Such a sensor can directly rectify a microwave field into a dc voltage signal using the Seebeck effect. Thanks to the high conversion efficiency of the microwave rectification in MTJs, the microwave power sensitivity of the spintronic sensor is on the order of 1–10 mV/mW. This high sensitivity allows the sensor to directly measure the coherent spatial scattered microwave field distribution, which gives it the ability to non-destructively detect hidden objects down to a few wavelengths in size.

Physical Mechanism and Theoretical Foundation of Ambient RF Power Harvesting Using Zero-Bias Diodes

Estimating the amount of harvestable ambient RF and microwave power from the omnipresent electromagnetic sources is of vital importance when designing a wireless device that makes use of ambient microwave power harvesting (AMPH) as a power source. This paper studies and looks into the underlying RF and microwave rectification mechanism at low input ambient power levels, specifically -30 dBm and below. A fundamental theory is formulated and developed, which is able to correctly predict the efficiency of a rectifier including the effects of matching network insertion losses through an easy-to-understand analytical model. The suggested model provides a direct design guideline in determining and choosing the optimal diode for a predetermined application. Based on the developed theoretical framework, the diode characteristics that have a direct impact on the microwave power conversion efficiency are discussed in detail. Three different Schottky diode rectifiers were designed on the basis of the tools described in this paper, thereby validating the proposed model and highlighting the influence of critical diode parameters on its performances. The measured results are then compared with those predicted by the proposed model and state-of-the-art microwave power rectifiers, showing a good model accuracy and also a 10% improvement in the rectifying efficiency for the low input power levels of interest.

Coupled Resonance Energy Transfer Over Gigahertz Frequency Range Using Ceramic Filled Cavity for Medical Implanted Sensors

A wireless power transmission (WPT) system based on a magnetically coupled coil-based resonance is the predominating technology in most WPT applications. In the field of implantable medical devices, however, the energy transfer operation is highly limited by the size of the receiver and also the loss properties of dispersive human tissue. Since the low gigahertz range has been considered as the optimal transfer frequency, an alternative to the lossy coil resonator should be studied and developed. In this paper, an original transmitter solution is presented that considers the needs for strong magnetic dominant near-field and weak far-field radiation even at low gigahertz frequency. A half-closed partially ceramic-filled cavity resonator is described on the basis of an accurate, but analytical model. Design parameters are also studied using a full-wave simulation software package and measurement results of a resonator-to-resonator transfer scheme are presented, which show a good agreement with simulation results. An efficiency above 65% can be obtained within the distance comparable to the diameter of the resonator (60.5 mm) in this case study. Subsequently, energy transmission between the proposed cavity resonator and a small-sized copper coil of 3 mm of diameter is investigated. Measurement results show that the efficiency is above 34% within 20 mm and above 8.2% within 40 mm, which is much higher than the conventional coil-to-coil transmission scheme.

Breaking the Efficiency Barrier for Ambient Microwave Power Harvesting With Heterojunction Backward Tunnel Diodes

Harvesting low-density ambient microwave power as an alternative power source for small ubiquitous wireless nodes has been proposed in recent papers discussing emerging technologies like the Internet of Things and Smart Cities. However, a literature review of the state-of-the-art Schottky diode based microwave rectifiers shows that a maximum efficiency has been reached for such devices operating in the low-power regime, as is the case for ambient microwave power-harvesters. This work examines the underlying physical mechanisms responsible for this RF-to-dc power conversion efficiency limitation, and explores a high I-V curvature backward tunnel diode to overcome this efficiency limitation. Measurements of the 2.4 GHz RF-to-dc power conversion efficiency at -40 dBm input power demonstrates that the backward tunnel diode outperforms the HSMS-285B Schottky diode by a factor of 10.5 and the Skyworks SMS7630 by a factor of 5.5 in a lossless matching network scenario. A prototype built using a new GSG probe embedded with a matching circuit showed a total power conversion efficiency of 3.8% for -40 dBm input power and 18.2% for -30 dBm input power at 2.35 GHz.

Overcoming the efficiency limitation of low microwave power harvesting with backward tunnel diodes

This paper explores, for the first time, the use of high responsivity heterostructure backward tunnel diodes to enhance the conversion efficiency of ambient microwave power harvesters. Progress in advancing the performance of low power rectifiers has been slowed because the maximum possible efficiency using Schottky diodes has been reached. Measurements of RF-to-DC conversion efficiency at -40dBm/2.4GHz are reported in this paper, showing that the backward diode outperforms the HSMS-285B Schottky diode by a factor of 10.5 and the Skyworks SMS 7630 by a factor of 5.5. A narrowband rectifier circuit was designed, fabricated and tested, showing a total efficiency of 3.8% for a 100nW input RF power and 18.2% at 1μW input RF power, at 2.35GHz.

Towards millimeter-wave high-efficiency rectification for wireless energy harvesting

This paper introduces a simple dual diode rectifier circuit in microstrip technology operating at K-band towards millimeter-wave applications. The designed rectifier circuit has a special architecture that enables the separation of the DC component of the rectified wave from the data-related IF channel. Diode characteristics are discussed for efficiency enhancement which is involved in accurate system simulations. Optimization procedure is carried out in order to maximize the RF-to-DC conversion efficiency. A measured efficiency of 40% for 35 mW input power is achieved for the designed circuit, showing an improvement in efficiency in comparison with previous works. The circuit presents potential applications in the design of integrated microwave and millimeter-wave systems for wireless power transmission and energy harvesting.

Hybrid Power Harvesting for Increased Power Conversion Efficiency

This work presents a cooperative power harvesting scheme, collecting power from two or more independent ambient energy sources into a single non-linear component. It is shown that the interaction of uncorrelated signals in a nonlinear device can greatly improve the conversion efficiency of the harvester. This letter proposes and studies a hybrid power harvester working with both microwave radiation and mechanical vibration excitations. Up to 6 dB gain on the harvested power is obtained for excitations in the range of maximum -40 dBm input power, when compared to a single excitation and standalone harvester working at the same input power level.

High sensitivity microwave detection using a magnetic tunnel junction in the absence of an external applied magnetic field

In the absence of any external applied magnetic field, we have found that a magnetic tunnel junction (MTJ) can produce a significant output direct voltage under microwave radiation at frequencies, which are far from the ferromagnetic resonance condition, and this voltage signal can be increase by at least an order of magnitude by applying a direct current bias. The enhancement of the microwave detection can be explained by the nonlinear resistance/conductance of the MTJs. Our estimation suggests that optimized MTJs should achieve sensitivities for non-resonant broadband microwave detection of about 5000 mV/mW.