Transistors - Page 9

Researchers at the University of Wisconsin-Milwaukee will be presenting a graphene-based sensing platform for real-time, low-cost detection of various water contaminants at the AVS's 64th International Symposium & Exhibition, being held Oct. 29-Nov. 3, 2017, in Tampa, Florida. The new sensor detects heavy metals, bacteria, nitrates and phosphates.

The sensor works by placing graphene-based nanosheets that are semiconducting between an electrode gap. The electrical conductivity of the graphene material changes with the binding of substances, called analytes, to its surface and their chemical constituents are identified and measured. "The magnitude of the conductivity change can be correlated to the concentration of analyte, and the technology also involves the functionalization of the graphene material surface with specific probes that can target a specific analyte," said the researchers.

Researchers at Chalmers University have developed a flexible detector for terahertz frequencies (1000 gigahertz) using graphene transistors on plastic substrates. It is said to be the first of its kind, and can extend the use of terahertz technology to applications that require flexible electronics, like wireless sensor networks and wearable technology.

At room temperature, the translucent and flexible device detects signals in the frequency range 330 to 500 gigahertz. The technique can be used for imaging in the terahertz area (THz camera), but also for identifying different substances (sensor). It may also be of potential benefit in health care, where terahertz waves can be used to detect cancer. Other areas where the detector could be used are imaging sensors for vehicles or for wireless communications.

A common challenge when attempting to make a graphene-based sensor is the high levels of electronic noise that are caused, reducing its effectiveness. In a recent work, an international team of researchers proposed a graphene-based semiconductor device that reduces electronic noise when its electric charge is neutral (referred to as its neutrality point). The group achieved this neutrality point without the need for bulky magnetic equipment that had previously prevented these approaches from being used in portable sensor applications.

In a proof-of-concept device, the researchers used their new sensing scheme to detect HIV-related DNA hybridization at picomolar concentrations. The team fabricated a charge detector out of graphene that can detect very small amounts of charges close to its surface. The sensing principle of the device relies on charge species detection through the field-effect, which brings about a change in electrical conductance of graphene upon adsorption of a charged molecule on the sensor surface.

Scientists at Rutgers University-New Brunswick have found a way to control the electrons in graphene, paving the way for the ultra-fast transport of electrons with low loss of energy in novel systems. "This shows we can electrically control the electrons in graphene," said a professor in Rutgers' Department of Physics and Astronomy. "In the past, we couldn't do it. This is the reason people thought that one could not make devices like transistors that require switching with graphene, because their electrons run wild."

This new work might make it possible to realize a graphene nano-scale transistor, the team said, which would be an important step towards an all-graphene electronics platform. The team managed to control electrons by sending voltage through a microscope with an extremely sharp tip, also the size of one atom, which offers 3-D views of surfaces at the atomic scale. The microscope's sharp tip creates a force field that traps electrons in graphene or modifies their trajectories, similar to the effect a lens has on light rays. Electrons can easily be trapped and released, providing an efficient on-off switching mechanism, according to the team.

Researchers with Johns Hopkins University and MIT have shown a way to cause flat sheets of graphene to self-fold into 3D geometric shapes. The group explains how they prepared the sheets and then used heat to cause them to fold. The ability to create 3D objects from sheets of graphene can advance opportunities in fields like sensors, wearables and more.

In their work, the researchers developed a micro-patterning technique that leads to the flat graphene sheets bending along predesignated lines when heat is applied, causing the sheet to form into shapes. The new method not only preserves the intrinsic properties of the graphene, but it was also found that the creases can cause a band gap in the graphene, which can be extremely useful.

The team describes how honey produces a nanometer-sized electric double layer at the interface with graphene that can be used to gate the ambipolar transport of graphene. "As a top-gate dielectric, water is much too conductive, so we moved to sugar and de-ionized water to control the ionic composition in hopes we could reduce conductivity," the team explained. "However, sugar water didn't work for us either because, as a gate-dielectric, there was still too much leakage current..... We decided to drop-cast honey on graphene to act as top-gate dielectric— I thought maybe the honey would mimic dielectric gels I read about in literature. To our surprise—everyone said it's not going to work—we tried and it did".

Researchers at the U.S. Department of Energy’s Ames Laboratory successfully manipulated the electronic structure of graphene, which may enable the fabrication of graphene transistors that could be faster and more reliable than existing silicon-based transistors.

The researchers were able to theoretically calculate the mechanism by which graphene’s electronic band structure could be modified with metal atoms. The work will guide experimentally the use of the effect in layers of graphene with rare-earth metal ions sandwiched (intercalated) between graphene and its silicon carbide substrate. Since the metal atoms are magnetic, the additions can also modify the use of graphene for spintronics.

Researchers at Chalmers University, affiliated with the Graphene Flagship, have devised a graphene-based spin field-effect transistor with the ability to function at room temperature. The team used the spin of electrons in graphene and similar layered material heterostructures to fabricate working devices in a step towards combining memory devices and the logic of spintronics.

The researchers demonstrated that the spin characteristics of graphene can be electrically regulated in a controlled way, even at an ambient temperature. In addition to possibly unlocking various probabilities in spin logic operations, this study also enables integration with magnetic memory elements in a device unit. If further advancements can assist in the production of a spin current without the need for charge flow, the amount of power needed will be considerably reduced, resulting in highly versatile devices.

Researchers at the University of Central Florida, the University of Texas at Dallas and other collaborators have designed a graphene-based transistor which could be used to create an all-carbon spin logic design with the potential to someday lead to computers that are a thousand times faster and use a hundredth of the power.

The team found that by applying a magnetic field to a graphene ribbon, it is possible to change the resistance of current flowing through it. For this device, the magnetic field is controlled by increasing or decreasing the current through adjacent carbon nanotubes. Increasing or decreasing the strength of the magnetic field would also increase or decrease the flow of current through this new kind of transistor, much like a valve controlling the flow of water through a pipe.

An international team of scientists collaborating in the Graphene Flagship project has developed a graphene-based transistor that reportedly outperforms previous state-of-the-art ones. The team utilized a thin top gate insulator material to optimize operational properties like maximum oscillation frequency, cutoff frequency, forward transmission coefficient, and open-circuit voltage gain, realizing devices that show prospect of using graphene in a wide range of electronic applications.

Graphene lack of a bandgap hinders its use in electronics since it causes an inability to switch the transistors off, effectively rendering the 0 state in digital logic inaccessible. However, many analog applications do not require a bandgap; The team explains that the only undesired side-effect of using GFETs in analog circuits is a poor saturation of the drain current, which prevents high-gain operation. The researchers have now succeeded in improving saturation by optimizing the dielectric material (AlOx) that is used to electrically insulate the top gate of the GFET. An improved quality of gate dielectric resulted in strong control over carriers in the graphene channel, yielding overall performance benefits.