Radio Frequency Technology and GrapheneMar 11, 2019 | ACS MATERIAL LLC
Graphene is on the tip of every radio frequency researcher’s tongue and for good reason. Electronic technologies are speeding towards increasingly faster and smaller computing, communication and automation. These advancements have required silicon-based transistors to become more and more miniature. Today’s circuitry is smaller, faster and more efficient than ever before, but it’s widely believed that the drive for smaller and more powerful has a natural technological limit: current materials cannot infinitely get both smaller and faster. To continue on this path, new materials are needed.
Enter graphene. Graphene appears to many as an ideal material for use in future electronics. Graphene has the potential to revolutionize RF technologies and pave the way for significant advancements in all RF systems, including low-noise amplifiers, frequency multipliers, mixers and high-speed radiometers. With graphene, many researchers believe we can achieve previously unimagined rates of density, speed and performance from computers, communication and automation.
What is Graphene and Why is it Potentially Ideal for RF Applications and Systems?
Very simply, graphene is a single layer of graphite. Graphite is an allotrope of carbon, which means that, while it’s entirely composed of carbon, its atoms are arranged in a unique way, giving it unique properties. When a single layer of graphene is removed from graphite, the result is a single layer of carbon atoms arranged hexagonally that behaves much differently than the original graphite or any other carbon-based material. At one atom thick, graphene is the world’s first truly 2-dimensional material. This remarkable substance has been attracting significant attention since 2004; it is thinner than paper and stronger than steel, flexible, transparent, conductive and impermeable to liquids and gases.
Among its many extraordinary characteristics, graphene exhibits many hallmarks of a material that is ideal for RF electronic applications. For one thing, graphene has shown high carrier mobility rates at room temperature, rates that are roughly 100 times greater than that of silicon and 10 times greater than the latest and greatest high-speed lattice-matched semiconductor materials. Graphene also has a saturation velocity estimated to be 5 times greater than Si MOSFETs (metal oxide semiconductor field-effect transistor) and it’s expected that graphene has a large on-state current density and transconductance per gate capacitance when compared to standard Si. Additionally, graphene FETs (field-effect transistors) have a distinctive ambipolar structure that can be utilized to create high-frequency multipliers, mixers, and radiometers. Graphene has the potential to offer exceptional switching characteristics and short-circuit current gain cut-off frequencies and it’s anticipated that graphene’s thin 2D channel layer can provide the ultimate path for vertical scaling of transistors with minimal short-channel effect. In short, single-atom-thick graphene has high carrier mobility, high saturation velocities, and high critical current densities, making it possible to create ultra-high-speed, ultra-small transistors. While there are still numerous challenges to overcome, it’s clear that graphene offers the potential for a number of unique and exciting device and circuit applications.
What’s been Tried with Graphene So Far?
Researchers and engineers have been developing and testing various graphene FETs and RF transistors. The first graphene FETs were demonstrated in 2007 and 2008 and were developed on the wafer scale using epitaxial graphene on graphitized hexagonal SiC substrates. Since these first efforts, graphene FETs have been demonstrated on a variety of substrates, including graphene on Si, graphene on SiO2, boron nitride substrates, and on harder, diamond-like carbon substrates. Graphene FETs are made by placing a micro-sized single-layer graphene channel between drain and the source. These graphene channels have demonstrated unprecedented sensitivity and conductivity; since the entire channel is only one atom thick, the entire channel is exposed to any and all molecules in the environment.
Epitaxial graphene RF transistors were first demonstrated in 2009. Standard transistors are made from silicon or more expensive materials such as indium phosphide. At similar voltages, graphene transistors allow electrons to zip around 100 times faster than silicon and 10 times faster than the rare and expensive indium phosphide. Graphene transistors are also more energy efficient than other transistors and potentially less expensive to produce. And, when you consider that graphene is flexible, strong and transparent, it’s not hard to see that graphene transistors will not only increase the speed and performance of technology, but they will also change the way technology looks and feels.
What are the Challenges with Graphene?
While it’s certain that graphene offers new opportunities, it’s also certain that significant challenges remain to be overcome. The future success of graphene FETs and RF transistors depends on the following:
- The ability to grow graphene on a large scale. Graphene is produced in two ways, either by a process called chemical vapor distribution, (CVD) or by exfoliation. Exfoliation is the easiest and least expensive method and simply requires a single layer of graphene to be peeled off of graphite using tape. This method works great for extracting small samples but cannot produce large sheets. CVD can create large volumes of graphene but can be expensive. During CVD, a carbon source is first decomposed at very high temperatures and then deposited on a substrate where a single layer of graphene eventually forms. The cost of the resulting graphene is a function of the cost of the original material, the substrate, and the transfer process. While it’s true that a layer of graphene can be created on a small silicon wafer in less than 5 minutes, it’s also true that it’s difficult to create graphene in large volumes cost effectively. Currently, the economies of large scale production are not in favor of graphene.
- The ability to produce graphene without defects. Both mechanical exfoliation and CVD have the potential to deposit chemical impurities that affect the material’s performance. Even the smallest presence of impurities could disrupt the single layer of atoms within a graphene sample.
- The ability to engineer a bandgap. Graphene is a phenomenal conductor. Even when the switch is off, electrons get through. Graphene does not have a bandgap, so technically, it cannot be switched off. Solutions to this dilemma involve treating the material as a semiconductor rather than a metal and limiting applications to those involving electron mobility. Chemically modified graphene (CMG) is also a possible solution. Some researchers have proposed that hydrogen deposited onto graphene could create stripes of non-conductive carbon atoms, forming a bandgap. Any way you look at it, there is still a lot of work to be done in this area to continue expanding the versatility of graphene.
While the progress from discovery to a marketable product is typically slow, graphene holds great and profitable promise. Graphene continues to capture the imaginations of researchers, scientists and engineers. Its potential applications are vast and constantly growing; graphene undoubtedly has the potential to change the world as we know it.