May 23, 2013
Graphene becomes bistable
Physicists at the universities of Manchester and Nottingham in the UK have discovered resonant tunnelling and strong negative differential conductance in graphene transistors. The new finding could help make high-speed and "bistable" logic devices from graphene and other two-dimensional crystalline structures.
Graphene and other 2D crystals – such as boron nitride, metallic dichalcogenides and layered oxides – can be stacked together to form a novel class of heterostructured materials that show promise for a range of device applications. By fine tuning the stack compositions, researchers may be able to create materials with novel electronic and optical properties that are much better than those of conventional semiconductors.
The Manchester–Nottingham team has now made a resonant tunnelling transistor device in which charge carriers (electrons and holes) tunnel through a 1 nm thick boride nitride barrier layer sandwiched between two graphene sheets that act as source and drain electrodes. The sandwich structure is mounted on an oxidized layer of doped silicon, which acts as the third electrode, the gate. When a bias voltage is applied across the electrodes, current flows through the boron nitride thanks to quantum mechanical tunnelling of electrons. The researchers control the current flowing through the transistor by changing the voltage applied to the three electrodes.
Graphene is a single atomic layer of carbon atoms arranged in a periodical honeycombed lattice. The material has a unique energy band structure that is very different from those in conventional semiconductors and metals. Electrons travel through the plane of graphene sheets as if they have no rest mass with speeds as high as 1 million m/s. The conduction and valence bands of graphene are cone-shaped and meet at the so-called Dirac point – named after Paul Dirac whose theory of relativistic quantum mechanics can be used to understand and model how massless fermions behave in 2D materials.
By tuning both the bias and gate voltages in their transistor, the researchers were able to align the energies of the Dirac points of the two graphene electrodes, something that allows the electrons in the structure to tunnel through efficiently while conserving energy and momentum. "When the device is tuned to this resonant condition, the current flowing though the device is at a maximum," explains team member Mark Fromhold. "And when the bias voltage is increased beyond this ‘tuned-in’ value, the resonant effect breaks and the current decreases sharply."
Negative differential conductance
A device in which the current decreases in such a way with increasing voltage is said to show negative differential conductance, he told nanotechweb.org, and this is phenomenon is not only of fundamental interest to physicists but also to electronics device engineers.
"Combining devices with negative differential conductance with other components to form a resonant circuit could make it possible for us to oscillate the current at very high frequencies – in the terahertz region of the electromagnetic spectrum," says Fromhold. "Terahertz frequencies are above the frequency range of conventional radar and below that of infrared heat radiation, so devices operating at this frequency are in great demand for applications in medicine and for security/surveillance systems."
The unusual dependence of current with bias voltage can also be exploited to create a fast "bistable" circuit in which the current flow can take on the same value for two different applied voltages, he added. This effect could find future applications in logic chips made from graphene and other 2D crystalline materials.
The present work is detailed in Nature Communications.
About the author
Belle Dumé is contributing editor at nanotechweb.org.