‘Mexican hat’ in graphene bilayer enables ultralow-power operation of processor chips

The most important consequence of reduced power consumption is the increase in performance which could be as high as two orders of magnitude

MOSCOW, May 16. /TASS/. Russian and Japanese scientists have developed graphene-based transistor and demonstrated with modeling the feasibility of its ultralow-power operation, the Moscow Institute of Physics and Technology (MIPT) said on Monday.

The most important consequence of reduced power consumption is the increase in performance which could be as high as two orders of magnitude. These record values are possible due to the unusual dependence of electron energy on momentum in graphene bilayer, which resembles a Mexican hat.

"Under optimum conditions, the current in graphene bilayer tunnel transistor can be changed by thirty thousand times with a gate voltage swing of only 150 millivolts", MIPT said. «Such a small operating voltage means that the novel transistor requires less energy for switching. At a lower power, the heating of electronic components is reduced, which means that they are able to operate at a higher clock speed - not one gigahertz, but ten for example, or even one hundred," said Dmitry Svintso, the head of the MIPT Laboratory of 2D Materials’ Optoelectronics.

A) The “mexican hat”-like dependence of electron energy on momentum in graphene bilayer (left) and the energy dependence of its density of states (right). Panel (B) highlights with red the electron states participating in tunneling in graphene bilayer (left) and in a semiconductor with common parabolic bands (right)

The realization of transistors capable of switching at low voltages (less than 0.5 volts) is one of the greatest challenges of today’s electronics. Tunnel transistors are among the most promising candidates to solve this problem. However, in most semiconductors the tunnel current is very small, and thus the transistors based on these materials cannot be used in real circuits. The author of the paper have shown that these limitations can be bypassed with a new design for a tunnel transistor based on bilayer graphene.

"Bilayer graphene represents two sheets of graphene attached one to another with intermolecular van der Waals bonds. It is as easy to produce as monolayer graphene, but due to the unique structure of its electronic bands, it is a highly promising material for low-voltage tunneling switches," said Svintsov.

Structure of the proposed transistor: bilayer graphene (red) is transferred onto the silicon dioxide layer (SiO2 ) or grown on the hexagonal boron nitride (hBN) substrate. A thin dielectric ZrO2 (2 nm) electrically isolates the transistor channel from the control gates. The side gates provide electrically doped contacts, the central gate operates over the transparency of the tunnel barrier

The electronic bands of bilayer graphene, which are the allowed energies at a given value of momentum, resemble the "Mexican hat". It turns out that the density of electrons near the edges of the "Mexican hat" tends to infinity - this is called a van Hove singularity. Upon the application of a tiny voltage to the gate of a transistor, a huge number of electrons at the edges of the "Mexican hat" simultaneously begin to tunnel. This causes an abrupt change in the current with the application of a small voltage, and this low voltage, in turn, results in the record low power consumption.

The researchers point out that until recently van Hove singularity in bilayer graphene was barely noticeable. In other words, the edges of the "Mexican hat" looked shabby due to the low quality of the samples. Nowadays, the graphene samples on hexagonal boron nitride (hBN) substrates are of much better quality, and sharp van Hove singularities in these samples are readily observed experimentally with scanning probe microscopy and infrared absorption spectroscopy.