Feb 3, 2012
Graphene transistor goes vertical
Graphene is highly conducting and thus ideal for electronic applications. However, its extreme conductivity can also be a problem because devices made from the material remain conducting even when switched off. Researchers at the University of Manchester have now taken a step forward in overcoming this problem by making a new type of transistor from graphene that contains layers of boron nitride or molybdenum disulphide sandwiched between graphene sheets. The layers act as vertical tunnelling barriers that minimize current leakage – even at room temperature.
Graphene consists of a planar single sheet of carbon arranged in a honeycomb lattice and has attracted much attention since it was discovered in 2004 thanks to its unique electronic and mechanical properties. The material could even replace silicon as the electronic material of choice in the future thanks to the fact that electrons travel ballistically through it at extremely high speeds.
However, there is a big problem. Integrated circuits should not consume electricity when they are switched off, but devices made from graphene continue to conduct even in their best off state. This not only wastes power but also means that the devices cannot be packed onto computer chips because the electric current running through the graphene would melt the chips almost immediately.
The reason why the material behaves this way is because, unlike the semiconductor silicon, graphene has no gap between its valence and conduction bands. Such a bandgap allows a material to switch the flow of electrons on and off. Researchers have proposed various schemes to overcome this problem – for example, by using nanoscale ribbons or quantum dots, or chemically modifying graphene to make it semiconducting. Although both schemes work in principle, opening a band gap in graphene in this way also damages the material so much that finished devices no longer show either ballistic transport or high electron mobilities.
Now, a team led by Leonid Ponomarenko working with Nobel Laureates Kostya Novoselov and Andre Geim has made a new type of device from graphene, a vertical field-effect tunnelling transistor. The device is the first ever made from graphene that can be properly switched on and off, despite the absence of an energy gap in the material's band structure.
The transistor is made of two graphene sheets sandwiched together with atomically thin insulators such as boron nitride (BN) or molybdenum disulphide (MoS2), which act as barriers for electrons tunnelling from one layer of graphene to another. The advantage of this type of structure is that the current flowing perpendicular to the layers of the insulating material – that is, the tunnelling current – can be controlled with an external electric field. "Technically, this is because the electrons induced in graphene by the external field have a higher probability for tunnelling and the number of such electrons increases with the field," explains Ponomarenko.
Although any insulator can be considered as a tunnelling barrier, it is only when the barrier is a few atoms thick that the tunnelling current can be easily measured. BN and MoS2 are ideal for use in this respect because extremely thin flakes of the materials can be produced using the same "sticky tape" method used to obtain graphene.
The device works thanks to a unique feature of graphene whereby an external voltage can strongly change the energy of the tunnelling electrons, says Ponomarenko. "I think our work now opens the way to create graphene integrated circuits," he told nanotechweb.org.
"The demonstrated transistor is important but the concept of such layer assembly is probably even more so," added Geim. Novoselov agrees: "The tunnelling transistor is just one example of the inexhaustible collection of layered structures and novel devices that could now be created by such assembly."
The team says that it will now look at whether graphene can be combined with other 2D materials. "It will also be important to know how our tunnelling transistors behave when their lateral size is reduced to the nanometre scale and to find out the highest frequencies at which the devices can operate," stated Ponomarenko.
The current work was published in Science.
About the author
Belle Dumé is contributing editor at nanotechweb.org.