Physicists from the Moscow Institute of Physics and Technology and colleagues from the Moscow State Pedagogical University and the University of Manchester have created a highly sensitive terahertz radiation detector based on the tunnel effect in graphene.
The sensitivity of the device already exceeds commercially available analogues based on semiconductors and superconductors, which opens up prospects for applications of the graphene detector in wireless communications, security systems, radio astronomy and medical diagnostics. The results of the study are published in the journal Nature Communications.
The transmission of information in wireless networks is based on the transformation of a continuous high - frequency wave into a sequence of segments-bits of information. This technique is called radiation modulation. To transmit information faster, you need to increase the modulation frequency. However, it is necessary to synchronously increase the frequency of the carrier radiation. If the usual FM radio uses signals at frequencies of hundreds of megahertz, then the carrier frequency of the Wi-Fi transmitter is already about five gigahertz, and for mobile transmission systems of the 5G generation, this frequency reaches twenty gigahertz. This is far from the limit, and a further increase in the carrier frequency promises a proportional increase in the data transfer rate. However, it is becoming increasingly difficult to detect signals with frequencies in the hundreds of gigahertz and higher.
The basic receiver used in wireless data transmission systems consists of a weak signal amplifier based on a transistor and a demodulator that "pulls" a useful sequence of bits from the ultra-high-frequency signal. This scheme, which originated in the era of radio and television, becomes ineffective at the desired frequencies for mobile systems in the hundreds of gigahertz. The fact is that most existing transistors do not have time to recharge at such a high frequency.
The "evolutionary" way to solve the problem is to increase the speed of the transistor. Most specialists in the field of nanoelectronics work in this direction. A" revolutionary " way to solve the problem was theoretically proposed in the early 1990s by physicistsBy Mikhail Diakonov andBy Mikhail Shur and implemented - including by a group of authors in 2018. This path consists in the rejection of the signal amplification by the transistor and the rejection of the demodulator. The transistor remains in the circuit, but its role now is different. It turns the modulated signal into a sequence of bits or voice information by itself, thanks to the nonlinear relationship between current and voltage.
In the current work, the authors proved that the detection of a terahertz signal is very effective in a special type of transistor, which is called a tunnel. To understand its operation, it is enough to recall the principle of an electromechanical relay, where the supply of current to the control contacts leads to a mechanical connection of the two conductors and the occurrence of current. In a tunnel transistor, the supply of voltage to the control contact - the gate-leads to the connection of the energy levels of the source and the channel, which, in turn, also leads to the flow of current. A distinctive feature of the tunnel transistor is its very strong sensitivity to the control voltage. After all, a small "detuning" of the energy levels is enough to interrupt the quantum-mechanical tunneling process. And already a small voltage on the control gate is able to "connect" the levels and initiate a tunnel current.
The created device is based on a two - layer graphene - a unique material in which the position of energy levels (more strictly-the band structure) can be controlled by an electric voltage. This allowed the authors to switch between the modes of classical and quantum tunnel transport within a single device with only a change in the voltage polarities at the control contacts. This capability is extremely important for accurate comparison of the detecting properties of a classical and quantum tunneling transistor.
The experiment showed that the sensitivity of the device in the tunnel mode is several orders of magnitude higher than the same value in the classical transport mode. The minimum signal detected by the detector against the background of noise already competes with the same indicator in commercially available superconducting and semiconductor bolometers. However, this is not the limit: the sensitivity of the detector can be increased further in "clean" devices with a low concentration of residual impurities. The developed detection theory, tested by the current experiment, shows that the sensitivity of the "optimal" detector can be a hundred times higher.
The results presented in this paper are an example of successful collaboration between several scientific groups. The authors note that this format of work allows them to once again receive world-class scientific results. For example, earlier this same team of scientists demonstrated how waves in the electronic sea of graphene can contribute to the development of terahertz technologies.
The study was supported by the Russian Science Foundation (grant No. 16-19-10557) and the Russian Foundation for Basic Research (Grant No. 18-29-20116 mk).
Information and photos provided by the MIPT press service
