When you think of how far semiconductor devices—particularly CMOS—have come in terms of speed, it can leave you dumbstruck. One day, though, silicon-based ICs will reach their speed limits. Researchers are, therefore, looking for new materials that can take over where silicon will leave off. One such material is graphene. Still in the research stage, graphene is showing promise for applications such as high-speed transistors, sensors, super capacitors, and solar cells.
Graphene sheets form a single-atom layer on silicon or other materials. Courtesy of IBM.
Graphene is carbon grown on substrates such as silicon, glass, and copper in layers that are just one atom thick. Thus, graphene sheets are two dimensional. The carbon atoms are arranged in hexagonal shape, like a honeycomb. When doped with impurities, graphene takes on some interesting properties.
For example, electrons can pass right through a graphene sheet without knocking into other electrons—a phenomenon called "ballistic transport," which means the material has very low resistance. Lower resistance results in lower voltage losses, which could lead to a new range of low-power electronic devices.
Graphene is being applied in the construction of RF transistors that could set new bandwidth records. IBM has announced the development of graphene transistors with a bandwidth of 100 MHz (Ref. 1). See links to papers on graphene research.
At first, graphene MOSFET transistors wouldn't work in digital applications because the single-layer sheet has no band gap between a MOSFET's gate and its source-drain channel. Without a band gap, a transistor won't turn off (Ref. 2) and would function as an analog device only. In June, researchers at Lawrence Berkeley National Labs announced that they had created a two-layer graphene device. With two layers, the researchers made a device with a controlled band gap, from 0 meV to 250 meV (Ref. 3). With that band gap, graphene transistors could work as digital devices.
As a sensor, graphene could, with the proper doping, detect the presence of specific chemical compounds. It's also showing properties that make it light sensitive. Thus, graphene could find use in solar cells or as infrared detectors (Ref. 4).
Super capacitors are yet another possible application for graphene. Experiments have shown that chemically modified graphene ranges in capacitance from 99 F/g to 135 F/g depending on the doping material. These values are high compared to dielectric capacitors but lower than batteries or fuel cells (Ref. 5).
As researchers experiment with graphene-based devices, they must characterize them. That requires measurements. Mary Anne Tupta, senior applications engineer at Keithley Instruments, often assists researchers who, when characterizing graphene transistors, measure a device's I-V curves with source-measure units. Some researchers are measuring the characteristics of the graphene sheets themselves. They may put just 1 pA of current through a sheet and measure its resistance. Many of the papers listed below discuss how researchers make these measurements.
References 1. "Made in IBM Labs: IBM Scientists Demonstrate World's Fastest Graphene Transistor," IBM, February 5, 2010. www.ibm.com. 2. Schwierz, Frank, "Graphene transistors," Nature Nanotechnology, published online May 30, 2010. www.nature.com/nnano. 3. "Bilayer Graphene Gets a Bandgap," Berkeley Lab, June 10,2009. www.lbl.gov. 4. Hwang, G., et al., "Graphene as Thin Film Infrared Optoelectronic Sensor," 2009 International Symposium on Optomechatronic Technologies. www.isir.upmc.fr/files/isot_09.pdf. 5. Stoller, M.D., et al., "Graphene-Based Ultracapacitors," American Chemical Society, Washington, DC, 2008. www.utexas.edu.
USB test switch for PXI
The Model 40-737 test switch from Pickering Interfaces lets you connect up to eight USB devices to a single PXI card. You can use the switch to automatically test USB peripherals. Each port's power is switched through software. www.pickeringtest.com.
Videos on noise figure measurements
Agilent Technologies has released a CD containing five videos called "Hints For Making Noise Figure Measurements." www.agilent.com.
Reference CD on nanoscale measurements
Keithley Instruments' "Nanotechnology Test Tutorials" CD contains technical papers, articles, and data sheets on how to make measurements on nanoscale semiconductor devices and carbon nanotubes. www.ggcomm/Keithley/Jun10_PR_NanoCD.html.
Graphene resources
The following links take you to scientific papers on graphene-based devices. The papers have been published by various scientific journals. (Links courtesy of Keithley Instruments.)
Agilent Technologies Inc. (NYSE: A) is the world's premier measurement company and a technology leader in communications, electronics, life sciences and chemical analysis...
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