The mechanical and electronic properties of two-dimensional materials make them promising for use in flexible electronics1–3. Their atomic thickness and large-scale synthesis capability could enable the development of ‘smart skin’1,3–5, which could transform ordinary objects into an intelligent distributed sensor network6. However, although many important components of such a distributed electronic system have already been demonstrated (for example, transistors, sensors and memory devices based on two-dimensional materials1,2,4,7), an efficient, flexible and always-on energy-harvesting solution, which is indispensable for self-powered systems, is still missing. Electromagnetic radiation from Wi-Fi systems operating at 2.4 and 5.9 gigahertz8 is becoming increasingly ubiquitous and would be ideal to harvest for powering future distributed electronics. However, the high frequencies used for Wi-Fi communications have remained elusive to radiofrequency harvesters (that is, rectennas) made of flexible semiconductors owing to their limited transport properties9–12. Here we demonstrate an atomically thin and flexible rectenna based on a MoS2 semiconducting–metallic-phase heterojunction with a cutoff frequency of 10 gigahertz, which represents an improvement in speed of roughly one order of magnitude compared with current state-of-the-art flexible rectifiers9–12. This flexible MoS2-based rectifier operates up to the X-band8 (8 to 12 gigahertz) and covers most of the unlicensed industrial, scientific and medical radio band, including the Wi-Fi channels. By integrating the ultrafast MoS2 rectifier with a flexible Wi-Fi-band antenna, we fabricate a fully flexible and integrated rectenna that achieves wireless energy harvesting of electromagnetic radiation in the Wi-Fi band with zero external bias (battery-free). Moreover, our MoS2 rectifier acts as a flexible mixer, realizing frequency conversion beyond 10 gigahertz. This work provides a universal energy-harvesting building block that can be integrated with various flexible electronic systems. Integration of an ultrafast flexible rectifier made from a two-dimensional material with a flexible antenna achieves wireless energy harvesting of Wi-Fi radiation, which could power future flexible electronic systems.

Two-dimensional MoS2-enabled flexible rectenna for Wi-Fi-band wireless energy harvesting.

New and emerging terahertz technology applications make this a very exciting time for the scientists, engineers, and technologists in the field. New sensors and detectors have been the primary driving force behind the unprecedented progress in terahertz technology, but in the last decade extraordinary developments in terahertz sources have also occurred. Driven primarily by space based missions for Earth, planetary, and astrophysical science, frequency multiplied sources have dominated the field in recent years, at least in the 2-3 THz frequency range. More recently, over the past few years terahertz quantum cascade lasers (QCLs) have made tremendous strides, finding increasing applications in terahertz systems. Vacuum electronic devices and photonic sources are not far behind either. In this article, the various technologies for terahertz sources are reviewed, and future trends are discussed.

Technology, Capabilities, and Performance of Low Power Terahertz Sources

Schottky diode-based terahertz frequency multipliers and mixers

We report on the design and performance of a broad-band, high-power 540-640-GHz fix-tuned balanced frequency tripler chip that utilizes four planar Schottky anodes. The suspended strip-line circuit is fabricated with a 12-/spl mu/m-thick support frame and is mounted in a split waveguide block. The chip is supported by thick beam leads that are also used to provide precise RF grounding. At room temperature, the tripler delivers 0.9-1.8 mW across the band with an estimated efficiency of 4.5%-9%. When cooled to 120 K, the tripler provides 2.0-4.2 mW across the band with an estimated efficiency of 8%-12%.

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A 540-640-GHz high-efficiency four-anode frequency tripler

A 540-640-GHz High-Efficiency Four-Anode

Design and optimization of Schottky varactor diode frequency multipliers for millimeter and submillimeter wavelengths are generally performed using harmonic balance techniques together with equivalent-circuit models. Using this approach, it is difficult to design and optimize the device and multiplier circuit simultaneously. The work presented in this paper avoids the need of equivalent circuits by integrating a numerical simulator for Schottky diodes into a circuit simulator. The good agreement between the calculated and published experimental data for the output power and conversion efficiency originates from the accurate physical model. The limiting effects of multiplier performance such as breakdown, forward conduction, or saturation velocity are discussed in view of the optimum circuit conditions for multiplier operation including bias point, input power, and loads at different harmonics. It is shown that the onset of forward or reverse current flow is responsible for the limitation in the conversion efficiency.

Modeling and design aspects of millimeter-wave and submillimeter-wave Schottky diode varactor frequency multipliers

The electrical and RF performance characteristics of submillimetre-wave Schottky diodes are investigated using an accurate physical model which combines drift-diffusion current transport with thermionic and thermionicfield emission currents imposed at the Schottky contact. The model includes self-consistently image force effect, tunneling transport, and current dependent recombination velocity at the Schottky contact. This physical simulator has been coupled to a harmonic balance simulator as a non-linear element. The integrated device-circuit simulator has allowed to study in detail the limiting factors of varactor operation. The contribution of large-signal capacitance excitation is discussed. The influence of external loads at different harmonics, bias and power level is also analysed.

Characterisation of Schottky Diode Performance by Numerical Simulation Coupled with Harmonic Balance

An integrated design philosophy which combines physical device modelling and circuit simulation is applied for the design of Schottky-diode frequency multipliers for millimeter-wave applications. The physical model is based on the drift-diffudion theory. It also includes self-consistently image force lowering, tunnelling transport, current dependent recombination velocity at the Schottky contact and impact-ionization. This coupling enables to concurrently optimize the device electrical and geometrical parameters together with achievable output power, conversion efficiency and the required loads at the specific harmonics.

J. Gismero

Papers

Modeling and design aspects of millimeter-wave and submillimeter-wave Schottky diode varactor frequency multipliers

Characterisation of Schottky Diode Performance by Numerical Simulation Coupled with Harmonic Balance

Optimization of millimeter frequency multipliers by incorporation of numerical modelling into circuit simulators.