Lower part shows the slightly misaligned crystal lattices of the two graphene electrodes. The two cones display the electronic band structure of the two electrodes in energy and momentum space, while the two gold curves are the energies at which the electrons can resonantly tunnel through the boron nitride barrier with energy and momentum conservation. Credit: Dr Mark Greenaway, University of Nottingham.
Lower part shows the slightly misaligned crystal lattices of the two graphene electrodes. The two cones display the electronic band structure of the two electrodes in energy and momentum space, while the two gold curves are the energies at which the electrons can resonantly tunnel through the boron nitride barrier with energy and momentum conservation. Credit: Dr Mark Greenaway, University of Nottingham.

Researchers have developed a new type of tunneling transistor comprising multilayers of graphene and hexagonal boron nitride (hBN). By sandwiching these two-dimensional materials in a stack, they have shown that these structures have new properties that could find future uses in high-frequency electronics.

The team, from the universities of Manchester, Lancaster and Nottingham in the UK, Russia, Seoul and Japan, has been investigating how the unique physical properties of graphene could be exploited to make electronic devices that could eventually replace silicon technology. Their novel sandwich structure involved an ultra-thin barrier of hBN placed between two single atomic layers of crystalline graphene – by applying a bias voltage across the two graphene electrodes, a current of electrons flows through the boron nitride barrier.

The electrons have insufficient energy to jump over the barrier, but can pass through it by a process called quantum tunneling. This process is also the mechanism by which alpha particles are emitted by radioactive atomic nuclei. As the graphene electrodes and boron nitride tunnel barrier are highly ordered and pure crystalline layers, the electrons can only tunnel through the barrier if their energy or momentum remains constant.

As one of the researchers, Laurence Eaves, told Materials Today, “The voltage applied between the two graphene electrodes provides a way of tuning the electrons for resonant tunneling, but we can also fine-tune it further by applying a gate voltage to the conducting silicon layer on which our graphene–boron nitride–graphene sandwich is mounted.” The resonance gives rise to a strong peak in the current at a particular voltage, and the current then decreases upon increasing the voltage further. It is this “negative conductance” effect that makes the device interesting as a high-frequency oscillator.

Although combining 2D materials into heterostructure stacks has previously been shown to offer materials capable of commercial application, this first demonstration of how their electronic behavior can be significantly altered by precisely controlling the orientation of the crystalline lattices, as reported in Nature Nanotechnology [Mishchenko et al. Nat. Nanotechnol. (2014) DOI: 10.1038/nnano.2014.187].

The future commercial exploitation of these devices is likely to require large-area wafers grown by epitaxial methods rather than the mechanical transfer approach used here. However, for now, the team hopes to achieve higher frequency operation by decreasing the electrical capacitance with redesigned electrical contacts applied to the graphene layers. Aligning the graphene layers even more accurately will further enhance the device’s properties.