May 19 2016
Researchers have created a different type of graphene-based transistor and through modelling they have confirmed that it consumes very low power when compared to other equivalent transistor devices. These results have been published in the Scientific Reports journal.
(A) Electron spectrum E(p) in bilayer graphene (left) and energy dependence of its density of states, DoS (right). At energy levels corresponding to the edge of the "Mexican hat" the DoS tends to infinity. (B) The red areas show the states of electrons that participate in tunneling in bilayer graphene (left) and in a conventional semiconductor with "ordinary" parabolic bands (right). Electrons that are capable of tunneling at low voltages are found in the ring in graphene, but in the semiconductor they are only found at the single point. The dotted lines indicate the tunneling transitions. The red lines indicate the trajectories of the tunneling electrons in the valence band. (Credit- Authors of the study)
The most significant effect of decreasing power usage is that it allows an increase in the clock speed of processors. As per the calculations, the rise could be as high as twice the times in magnitude.
"The point is not so much about saving electricity - we have plenty of electrical energy. At a lower power, electronic components heat up less, and that means that they are able to operate at a higher clock speed - not one gigahertz, but ten for example, or even one hundred," says Dmitry Svintsov, the corresponding author and head of MIPT's Laboratory of Optoelectronics and Two-Dimensional Materials.
Constructing transistors capable of switching at low voltages (less than 0.5V) is one of the biggest challenges of current electronics. Tunnel transistors are considered to be the most promising candidates in solving this issue. In contrast to traditional transistors, where electrons "jump" across the energy barrier, the electrons "filter" through the barrier in tunnel transistors because of the quantum tunneling effect. On the other hand, in majority of semiconductors, the tunneling current is very small and this prevents the usage of transistors based on these materials in real circuits.
The article’s authors who are researchers from the Moscow Institute of Physics and Technology (MIPT), the Institute of Physics and Technology RAS, and Tohoku University (Japan), suggested a novel design for a bilayer graphene based on tunnel transistor, and through modelling, they demonstrated that this material is a perfect platform for low-voltage electronics.
Graphene, produced by MIPT alumni, Sir Andre Geim and Sir Konstantin Novoselov, is a carbon sheet of single-atom thickness. Since it has two dimensions only, graphene’s properties, like its electronic properties, are completely different from that of three-dimensional carbon - graphite.
Bilayer graphene is two sheets of graphene that are attached to one another with ordinary covalent bonds. It is as easy to make as monolayer graphene, but due to the unique structure of its electronic bands, it is a highly promising material for low-voltage tunneling switches.
Dmitry Svintsov
Bilayer graphene bands, that is the accepted energy levels of an electron at a particular value of momentum, are in the form of a "Mexican hat" (evaluate this to most semiconductors bands that form a parabolic shape). The density of electrons that occupy spaces near the edges of the "Mexican hat" tends to infinity - this is termed as a van Hove singularity. By applying even a very small voltage to the transistor gate, a large number of electrons at the edges of the "Mexican hat" begin to channel simultaneously. This leads to a sharp change in current upon applying a minute voltage which is the reason for the record low power usage.
In their study, the scientists state that until recently, van Hove singularity was hardly visible in bilayer graphene, i.e., the boundaries of the "Mexican hat" were not clear because of the poor sample quality. Current graphene samples on hBN (hexagonal boron nitride) substrates are of higher quality, and prominent van Hove singularities have been experimentally demonstrated in the samples by utilizing infrared absorption spectroscopy and scanning probe microscopy.
A significant aspect of this proposed transistor is the application of "electrical doping" (the field effect) to produce a tunneling p-n junction. The intricate process of chemical doping needed while fabricating transistors on 3D semiconductors, is not required (and could even be harmful) for bilayer graphene. In electrical doping, extra electrons (or holes) crop up in graphene as a result of attraction towards doping gates positioned closely.
Under optimal conditions, a graphene transistor can alter the circuit current ten thousand times with a gate voltage swing of just 150mV.
This means that the transistor requires less energy for switching, chips will require less energy, less heat will be generated, less powerful cooling systems will be needed, and clock speeds can be increased without the worry that the excess heat will destroy the chip.
Dmitry Svintsov