Novel carbon nanotube-based transistors reach THz frequencies

Carbon nanotubes (CNTs), cylindrical nanostructures made of carbon atoms arranged in a hexagonal lattice, have proved to be promising for the fabrication of various electronic devices. In fact, these structures exhibit outstanding electrical conductivity and mechanical strength, both of which are highly favorable for the development of transistors (i.e., the devices that control the flow of current in electronics).

In recent years, several electronics engineers have started using CNTs to develop various electronics, including metal-oxide-semiconductor field-effect transistors (MOSFETs). MOSFETs are transistors that control the flow of current through a semiconducting channel utilizing an electric field applied to a gate electrode.

Notably, when arrays of CNTs are used to develop MOSFETs, they can operate at radio frequencies (RF), the range of electromagnetic waves that support wireless communication. The resulting MOSFETs could thus be particularly advantageous for the advancement of wireless communication systems and technologies.

Researchers at Peking University, Xiangtan University and Zhejiang University recently developed new CNT-based MOSFETs that can operate effectively at THz frequencies, which essentially means that they can switch or amplify signals that change over a trillion times per second. The new transistors, presented in a paper published in Nature Electronics, could open new possibilities for the development of ultra-fast wireless communication systems, as well as high-speed radar and computing systems.

“Films of aligned semiconducting carbon nanotubes could be used to build complementary metal–oxide–semiconductor field-effect transistors for digital integrated circuits and radio-frequency transistors for terahertz analog integrated circuits,” wrote Jianshuo Zhou, Zipeng Pan and their colleagues in their paper.

“However, the operating frequencies of such devices remain too low for potential application in the sixth generation of wireless communications. We report metal–oxide–semiconductor field-effect transistors that are based on aligned carbon nanotube films and have a cut-off frequency beyond 1 THz.”

Conventional silicon-based transistors deployed in existing electronics can reliably handle signals with frequencies up to 100–300 GHz. This means that they can switch or amplify signals up to a few hundred billion times per second.

In contrast, the new CNT-based MOSFETs developed by Zhou, Pan and their colleagues can handle signals beyond 1 THz (i.e., 1,000 GHz). This makes them suitable for the development of sixth-generation (6G) wireless technologies and high-speed communication systems.

“By optimizing gate structures and fabrication processes, we create devices with a gate length of 80 nm that have a carrier mobility of over 3,000 cm² V⁻¹ s⁻¹, as well as an on-state current of 3.02 mA µm⁻¹, a peak transconductance of 1.71 mS μm⁻¹ at a bias of −1 V, and a saturation velocity of 3.5 × 10⁷ cm s⁻¹,” wrote the authors.

“By introducing a Y-shaped gate, we also create devices with gate lengths of 35 nm that have an extrinsic cut-off frequency (f_T) of up to 551 GHz and a maximum oscillation frequency (f_max) of 1,024 GHz. Finally, we use devices with a gate length of 50 nm to fabricate mmWave-band (30 GHz) radio-frequency amplifiers that have a gain of up to 21.4 dB.”

To demonstrate the potential of their newly developed MOSFETs, the researchers used them to create radio-frequencies amplifiers, electronic circuits that increase the strength of signals. These amplifiers, which operate in the millimeter-wave band, were found to perform remarkably well, reliably increasing the strength of high-frequency (30 GHz) signals by over a hundred times.

Overall, the recent work by Zhou, Pan, and their colleagues highlights the potential of CNT arrays for the development of transistors that could support faster communications. In the future, the MOSFETs they developed could be improved and used to develop ultra-high-speed wireless transmitters and receivers, as well as powerful THz sensing technologies and high-speed electronic circuits.

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