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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

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-tounderstand 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.

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