Researchers Pencil In Graphene Transistors
Graphene's weird electrical properties allow for smallest transistor yet
IMAGE: Anna Demian/Randi Silberman
Graphene, found in pencil marks, is a candidate material for making future transistors. It's extracted from graphite crystals [right] using sticky tape.
The little smudges you leave behind whenever you use a pencil could be the key ingredient of the next revolution in computer circuitry, according to experts around the globe. Part of what shears off from the graphite in a pencil is a substance known as graphene, a one-atom-thick crystal with remarkable electrical properties that may overcome the physical limits silicon faces as transistors shrink to ever-smaller sizes.
Silicon's remarkable run as ruler of the chip world may be nearing an end as engineers eventually lose the ability to make faster silicon transistors by making them smaller. In the hunt for what comes next, carbon nanotubes have gotten a big chunk of the attention, but if the current explosion of research activity is any indication, it may be graphene that wins in the end. This spring saw a flurry of breakthroughs surrounding graphene, culminating in the creation of what may be the smallest transistor ever made--one atom thick by 10 to 50 atoms wide.
Like carbon nanotubes, graphene is a crystal structure of carbon atoms but arranged in a flat plane instead of a cylinder. The electrons in graphene behave as if they have no mass. Like photons--but unlike electrons in other materials--the electrons move at a constant speed, regardless of how much energy each one has.
A transistor built out of graphene, therefore, should operate much faster than a comparable one made from silicon. Michael S. Fuhrer, a physicist at the University of Maryland's Center for Nanophysics and Advanced Materials, recently showed that at room temperature electrons in graphene move at 200000 centimeters per second for every volt per centimeter of electric field, 100 times faster than in silicon. ”All other things being equal, that would translate into a 100 times faster transistor,” he says.
Graphene has been known for decades as a single plane of graphite, but it was only in 2004 that Andre Geim and Kostya Novoselov of the University of Manchester, England, were able to isolate it by the simple act of pressing a piece of tape to a graphite crystal and placing it on a silicon substrate. In April, the two researchers described their transistor, 10 to 50 atoms wide and built by etching a pattern into graphene.
The substance is not a natural choice for a transistor material, in that it has no electrical bandgap, so applying an external electric field doesn't block the transport of electrons. In other words, a graphene transistor would be hard to turn off. Geim and Novoselov overcame that obstacle by etching away some of the graphene to create narrow constrictions, which, in the odd physics of two-dimensional materials, produced an artificial bandgap.
Funding for graphene research is flowing, notably from the EU's Graphene-based Nanoelectronic Devices (GRAND) program, which among other things is looking at whether graphene would still work its wonders when integrated with the silicon CMOS process. In the United States, the Defense Advanced Projects Research Agency is funding graphene research in the quest for better RF circuits. Industrial heavyweights such as IBM are also exploring the material. The company constructed a graphene transistor last fall.
Graphene may find a home outside ICs as well. Its high electrical conductance could lead to more sensitive chemical sensors, and it could prove to be a cheaper, more flexible substitute for the indium tin oxide used as transparent electrodes in LCDs and touch screens. It may also allow batteries to pack in more energy.
Despite widespread excitement over graphene's potential, Maryland's Michael Fuhrer warns that the research is still in its early days. ”Right now we're at the stage where we've got this interesting material to play with,” he says, ”but I think it's all blue sky.”
Robert Westervelt, a Harvard University expert on how electrons behave in 2-D materials, doubts that graphene will entirely replace silicon computing devices, but he says the material could lead to specialty circuits that compute in new ways, perhaps using some characteristic of electrons other than charge. ”There are probably more questions than answers now, but it's got a lot of people excited,” he says. ”All of the rules are really changed about the ways the electrons behave.”