This paper reviews the evolution and the historical milestones/breakthroughs of electromagnetic energy conversion techniques over the years with an emphasis on low-density energy-harvesting technologies.
An intrinsic thermoelectric coupling effect in the linear response regime of magnetic tunneling junctions (MTJ) is reported. In the dc response, it leads to a nonlinear correction to Ohm's law. Dynamically, it enables a novel Seebeck rectification and second harmonic generation, which apply for a broad frequency range and can be magnetically controlled. A phenomenological model on the footing of the Onsager reciprocal relation and the principle of energy conservation explains very well the experimental results obtained from both dc and frequency-dependent transport measurements performed up to GHz frequencies. Our work refines previous understanding of magnetotransport and microwave rectification in MTJs. It forms a new foundation for utilizing spin caloritronics in high-frequency applications.
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.Index Terms-Backward tunnel diode, microwave power harvesting, microwave power rectification, rectenna, Schottky diode.
I. INTRODUCTIONT HE way the internet is perceived by its users is going through a significant transformation. Today, nearly two billion people have access to the internet [1], using it to browse websites, play games, work, send and receive e-mails and messages, among many other on-line applications. Most of these applications, however, share a common characteristic: an end-user terminal. Nonetheless an important revolution is in progress, in which ubiquitous objects will be more and more connected to the internet or other local networks, leading to the Internet of Things (IoT) [1], [2]. Some emerging applications of this new Manuscript
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