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«Mexican Hat» Makes Electron Density Reach Infinity

Scientists have developed a new type of transistor based on bilayer graphene, and proved with the help of simulation that it has a record-low power consumption in comparison with existing analogues. This is reported in an article published in the Scientific Reports journal. The most important consequence of the reduction of power consumption of transistors is the ability to increase the frequency of the processor. According to calculations, it can grow up to two orders of magnitude. These records are possible thanks to the extraordinary dependence of the electron energy on the momentum in bilayer graphene, which resembles a Mexican hat in its shape.

"The point here is not to save electricity - we have enough electricity. At lower power consumption electronic components heat less, and therefore can be operated at a higher speed - not one gigahertz, but, for example, 10 or even 100 ", - says the study's lead author, Head of the Laboratory of Optoelectronics of two-dimensional materials and teacher of General Physics Department of MIPT Dmitry  Svintsov.

Creating transistors capable of switching at low voltages (less than 0.5 volts) is one of the greatest challenges of modern electronics. The most promising candidates for the solution of problems are tunneling transistors. Unlike classical transistors, where electrons "jump" the energy barrier, in tunneling transistors electrons "leak" through the barrier thanks to the quantum tunneling effect. However, in most semiconductors the tunneling current is very small and it does not allow use tunneling transistors based on them in real circuits.

Dmitriy_Svintsov Dmitriy Svintsov who is leader of the Laboratory of Optoelectronics of two-dimensional materials at MIPT

The authors, researchers from MIPT, Physics and Technology Institute Russian Academy of Sciences and Tohoku University (Japan) proposed a new design of the tunnel transistor based on bilayer graphene, and with the help of simulation proved that this material is an ideal platform for low-voltage electronics.

Graphene, the amazing properties of which were opened by MIPT graduates Andrei Geim and Konstantin Novoselov, is a sheet of carbon just one atom thick. Due to the two-dimensional properties of graphene, including electric ones, it is radically different from the three-dimensional carbon - graphite.

"The two-layer graphene is two sheets, which are interconnected by van der Waals forces. Getting it is as simple as a single-layer graphene, but due to the unique structure of the energy bands, it is a very promising material for low voltage tunneling switches "- Svintov says.

Energy bands of bilayer graphene, ie allowed values of energy of an electron at a given value of momentum, have a shape of "Mexican hat" (Fig. 1A, for comparison, the energy bands of most semiconductors have the form of a paraboloid). It turns out that the density of electrons, which can be placed near the "Mexican hat" approaches infinity edges. This feature is called a van Hove singularity. By applying a small voltage at the gate of the transistor a huge number of electrons at the edges "Mexican hat" at the same time begin to tunnel. This leads to a sharp change in current upon application of a low voltage and the fact that very low voltage is used leads to an unprecedentally low power consumption.

In their paper, the researchers point out that until recently, van Hove singularity in bilayer graphene was barely noticeable. In other words, the edges of "Mexican hat" looked tacky due to the low quality of the samples. Modern samples of graphene on a substrate of hexagonal boron nitride (hBN) have much better quality, and availability of acute van Hove singularities in them is experimentally confirmed using scanning probe microscopy and infrared absorption spectroscopy.

graphene (A) dependence of the electron energy on the pulse in bilayer graphene, which resembles Mexican hat (left), and the energy dependence of the density of states (right). At the energy corresponding to the edge of the hat, the density `of electronic states (density of states, DoS) tends to infinity. (B) Red marked are states of electrons that are involved in tunneling in bilayer graphene (left) and in a semiconductor with a "normal" parabolic bands (right ). Electrons that can tunnel at low voltages lie on the ring in graphene, and the semiconductor zones with parabolic - only one point. The dotted line indicates the tunnel junctions. The red lines indicate the path of tunneling electrons in the valence bands (valence band).

Designed made by the authors of the transistor is unique for another reason: its creation does not require chemical doping of graphene. Chemical doping - is the dissolution of small amounts of a semiconductor in the other, which serves to increase the electrical conductivity. For example, the dissolution of phosphorus in silicon leads to the same effect as the dissolution of the salt in the water - the resulting mixture begins to conduct. Operation of doping is one of the most sophisticated microelectronics technology. Fortunately, the two-layer graphene is a good conductor of current itself; Moreover, variate its conductivity is not necessary to introduce foreign matter it is sufficient to energize the correct polarity to the so-called "doping gates" ( "doping gates" on Figure 2.).

Under optimum conditions, the graphene transistor can change the current in the circuit thirty five thousand fold when the gate voltage swing only 150 millivolts.

"This means that the transistor requires less energy to switch, require less energy chips, less heat is released, you need less powerful cooling system, and the processor speed can be increased without fear that excess heat will destroy the chip," - Svitsov says.

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