Graphene has become the darling of the postsilicon crowd in the eight years since Andre Geim and Konstantin Novoselov isolated it by ripping Scotch tape off a chunk of graphite. But there are other two-dimensional nanowonders. Molybdenum disulfide (MoS2), which can be pulled off a block of molybdenite through the same process, could offer new approaches to making high-speed logic circuits—on its own or in combination with graphene.
“We spent the last seven, eight years looking at how to make transistors out of graphene,” Geim says. “But there was an elephant in the room—that you can’t really switch off current in graphene.”
Graphene’s single atomic layer of carbon atoms allows electricity to flow rapidly, promising circuits that work far faster than silicon transistors. But it lacks a bandgap, meaning that it’s hard to shut off the flow, making the on and off states of digital logic nearly impossible. Several approaches—using nanoribbons, quantum dots, or double layers of graphene—have been tried, but they are difficult, poorly developed, and tend to undermine the speed advantage.
So Geim and his colleagues at the University of Manchester, in England, tried a different design. They built a field-effect transistor with a vertical heterostructure—two layers of graphene separated by MoS2 or boron nitride. Like graphene, those materials are considered two-dimensional because they’re as thin as they can be; MoS2, for instance, is a single atomic layer of molybdenum sandwiched between two single layers of sulfur. The material acts as a barrier, preventing charge from crossing. So ordinarily, the transistor is in the off state— no charge flows from one graphene layer to the other.
For the on state, the researchers raise the voltage of one of the graphene layers. That boosts the energy of the electrons in that layer, causing them to tunnel through the barrier into the other graphene layer. Since tunneling is by nature very rapid, the process should make for very fast circuits, Geim says. The device area could be scaled down to as small as the latest lithographic techniques allow, which is better than today’s silicon circuits, Geim claims.
Using boron nitride, the ratio between on and off was only about 50, but with MoS2 it was about 10 000, sufficient for some logic circuits. But Geim isn’t wedded to MoS2. The experiments, published in Science in February, were only a proof of concept, and there may be some material that works even better.