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A less developed but equally promising category of new materials is metamaterials. There are many definitions for exactly what a metamaterial is, but the root understanding is that metamaterials are man-made structures that exceed the capability of natural materials. This area could be exciting for microwave engineers, as the periodic array of sub-wavelength structures that make up a metamaterial exhibit EM responses that are not typically found in nature (Fig. 4). Among the potential results could be magnetic metamaterials with very low loss, magnetic materials with very low dispersion, negative permeability structures, negative permittivity structures, artificial impedance surfaces, and a host of other potentially beneficial effects that are ideal for future sensing and communication systems. Generally, the capabilities of these new materials could lead to size reductions, lower costs, and higher speeds--all high-demand benefits of the microwave industry.

RF Materials Can Open The Gates For Next-Generation Technology, Fig. 4

Some examples of metamaterial applications include frequency-selective surfaces (FSSs), compact multiband antennas, EM bandgap structures, resonant permittivity/conductivity sensors, precise hyperthermia tools, and strain sensors. An example of the unique EM behavior of several metamaterial arrays is shown in Fig. 3.

Raytheon is already working with several universities and industry partners to develop practical uses for these materials . Raytheon’s goal is to add capability while reducing the size of its EM products using metamaterial technology. For example, high-performance aircraft have less available real estate for wireless technology as the capabilities and performance of the aircraft increase. This reduction leads to the need for broader bandwidth, dual polarization, and wide-angle scanning capability from multi-function conformal-antenna structures. Current structures suffer from narrow scan angles and bandwidths, but metamaterial antenna enhancements could prove the deciding factor in realizing the high end requirements. In a collaboration with the University of Florida, Raytheon is developing reduced-size, dual-band, metamaterial Global Positioning System (GPS) antennas. Current technologies are limited by the narrowband function that is a feature of many resonant metamaterials. But this collaboration’s goal is to enhance the multiband capability of the GPS antennas.

RF Materials Can Open The Gates For Next-Generation Technology, Fig. 5

Electronic bandgap structures are made by using periodically placed grounded vias. In higher-dielectric-constant substrates, their use can result in an increase in the bandwidth and scan-angle performance of the microstrip patch antenna arrays that require such substrates. Through the use of metamaterial techniques like electronic bandgap structures, the surface waves that limit the scan-angle performance can be disrupted, thereby enhancing performance.

Metamaterials also are being heavily explored by the scientific community for their size reduction and unique sensing capabilities for medical applications. Currently, research efforts are being directed to solutions like a highly efficient dual-band antenna operating at the universal mobile telecommunication system (UMTS) band and the industrial, scientific, and medical (ISM) band. Efforts also are being directed toward developing microwave devices that could detect cancer cells with low cost and high precision. Because cancer cells have significantly different water content than normal tissue, they also have higher permittivity and conductivity. With a suitably sensitive biosensor array with metamaterial resonators, these diseased tissues could be precisely identified with a fast and non-invasive detection method. Additionally, high concentrations of microwaves are capable of heating internal tissue. A precise and directed emission of microwaves from a metamaterial lens with a negative-refractive index could be used to focus on a target area and induce hyperthermic death of the tissue.

The unusual resonant behavior of modern metamaterials and future wideband metamaterial structures can offer benefits to RF applications that could not be previously realized, as size and cost were prohibitive. It should be noted that current metamaterials do have a flaw in their narrow bandwidth capability. With adequate research and development advances in metamaterial structures, however, the future could offer highly compact, power-efficient RF structures with spectacular properties. Another category of materials that also offer benefits of size and cost reduction of RF electronics are carbon-based electronic materials.

Carbon-based electronics are standard electronic elements that are enhanced with high performing carbon-based materials that have only recently been discovered and successfully fabricated. The two most common carbon-based electronic materials discussed for RF applications are graphene nano-ribbons (GNRs; a monoatomic layer of carbon atoms as seen in Fig. 1) and carbon-nano-tubes (CNTs), a mono-atomic tube of carbon atoms (Fig. 5). These new materials are attracting significant attention from both researchers and engineers because of their extraordinary electrical/optical behavior. With their nanometer dimensions, superior strength, extremely high carrier mobility, semiconductor behavior, and predictions of up to terahertz performance, many experts consider GNRs and CNTs to be the next revolution in high-speed computer fabrication and RF electronics.

It is important to understand the similarities and differences between GNRs and CNTs in regard to their material properties and potential benefits for specific applications. CNTs exhibit one-dimensional (1D) carrier transport. This channeled electron motion reduces the scattering probability by directly increasing the current carrying capacity. It also lowers the thermal noise profile while increasing the size of the mean free path through the CNT. Models that predict terahertz performance have been derived from applications like employing CNTs as channels in a field-effect transistor (FET). The high carrier velocity of up to 80 million cm/s and very low quantum capacitance allow for a very fast-recovering, highly linear, and near-ballistic transport-based CNTFET. Such characteristics are ideal for future ultra-high-speed RF amplifiers, mixers, and switches.

GNRs also have exhibited extremely high mobility that reportedly ranges up to 100 times that of silicon (Si). In addition, they boast ambipolar transport properties, capability for electrostatic doping, and a thin/flexible structure that is a good fit for many RF applications. Among such applications are those with low-noise amplification and high-sensitivity as well as mixers and reconfigurable circuits.

Both carbon-based materials have drawbacks spanning fabrication to negative interface interactions. The atom-thick layer of carbon in GNRs and CNTs causes degradation of bandgap voltage, carrier mobility, Schottky barrier height, and effective charge density when embedded in a transistor structure in close proximity to other materials. There also are many fabrication difficulties to overcome with the creation of suitable carbon-based electronics. Current fabrication techniques degrade the overall performance of the materials while complicating integration into modern fabrication processes. Despite these challenges, companies like RFnano are already producing and working on the next stage of CNT implementations for the RF industry.

As for the research debates over which carbon-based material is better for ultra-high-frequency operations with FET devices, experiments with graphene field-effect transistors (GFETs) and CNTFETs have been performed by many labs. GFETs tend to have high output conductance, which lowers the internal voltage gain to a factor much lower than 1. This prevents GFETs from operating as high-gain circuits. CNTFETs have voltage gains slightly larger than 1. For example, a discrete amplifier built by MITEQ using CNTFETs fabricated by RFNano was detailed in the study, "L-band carbon nanotube transistor amplifier." This amplifier allegedly exhibited 11-dB power gain at 1.3 GHz and 6 dB power gain at 2 GHz. In a recent study by IBM, "Ultimate RF Performance Potential of Carbon Electronics," both the power-gain and current-gain cutoff frequencies of CNTFETs and GFETs were found to exceed 1 THz. Those cutoff frequencies are much higher than silicon-nanowire or gallium-arsenide (GaAs) semiconductors. It was also found that CNTFETs require lower biasing currents, are less sensitive to fabrication defects, and have higher cutoff frequencies than GFETs.

In closing, the technology for fabricating carbon-based electronics is still several years away from being incorporated into mainstream electronics. Given the promising experimental results of high-performing carbon materials, however, the semiconductor roadmap looks solid—even after the RF industry’s demands exceed the limits of Si or GaAs.

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