Researchers at the Peking University developed carbon nanotube (CNT) based RF-transistors that can operate at terahertz frequencies.
High frequency semiconductor devices are useful in microwave engineering, communication systems and other applications like this. For instance, a microwave power amplifying device includes an active element such as a high frequency FET, a passive element such as a resistor or a capacitor, and a circuit element such as a microstrip line that transmits a high frequency signal. In communication systems, high frequency FETs are used to amplify weak signals from the satellites at the receiver end.
One of the things that limits the high frequency operation of the semiconductor devices is the capacitance associated with them. For example, MOSFETs have an input capacitance which dictates how fast it can detect signals at its gate. The output capacitance and output resistance for a low-pass filter that dictates the overall frequency response of the circuit. And the speed of a semiconductor device can be enhanced by changing the material or fabrication process.
For next generation communication systems, devices that work at extremely high frequencies greater than 90GHz are required. These devices are radiofrequency devices. Currently silicon complementary metal-oxide-semiconductor (CMOS) field-effect transistors (FETs), and GaAs transistors are used as radiofrequency devices. But these devices can’t achieve high operating frequencies and also be easy to integrate within wireless communication technologies at the same time.
Due to their electrical properties, carbon nanotubes (CNTs) are the promising candidate for the development of the ultra-high frequency semiconductor devices. The devices made by single-wall carbon nanotubes have the potential to run at up to terahertz frequencies. Carbon nanotubes are as thin as an atom, and they present high electron mobility. As a result, the CNT based devices are much faster than silicon based devices. Moreover, they are also more power efficient than silicon based devices.
The speed of CNT based FETs, however, has been limited and lag behind the theoretical predictions. This unsatisfactory operation is due to misaligned semiconducting CNT arrays with a suitable density, high uniformity, high semiconducting purity and high carrier mobility. To overcome these challenges, researchers at the Peking University in China have recently fabricated new RF transistors based on aligned CNT arrays. They created these transistors using two distinct methods, a double-dispersion sorting and a binary liquid interface aligning process.
For fabrication of these transistors, the researchers used two processes known as electron beam lithography (EBL) and atomic layer deposition (ALD). They then completed preparing each functional layer of the device using other nano-fabrication equipment, via what is known as a top-down lift-off process.
The developed nanotube arrays have a density of approximately 120 nanotubes per micrometer, exhibiting a carrier mobility of 1,580 cm2V-1s-1 and a saturation velocity of up to 3.0×107 cm s-1. The newly developed CNT FETs achieved a high D.C. performance when operating at millimeter-wave and terahertz frequencies.
One of the primary functions of FETs in communication devices is amplification. To enable amplification of radio-frequency signals, the transistors created by the researchers rely on the transconductance amplification of the FET device. Moreover, their speed of operation depends on the carrier mobility in the device channel.
“The main advantages of our transistors can be summarized as three main points,” Lianmao Peng, one of the researchers who carried out the study, said. “Firstly, our devices based on high-density semiconducting CNT arrays show the strong on-state driving capability, leading to large transconductance and large current, which brings the strong RF signal amplification capability. Secondly, our CNT arrays exhibit high carrier saturation speed and high mobility, corresponding to the high current gain cut-off frequency (fT) and power gain cut-off frequency (fMAX).”
According to the researchers, the newly developed CNT based devices can reach desirable levels outlined by theoretical predictions. The researchers are now focusing on improving the performance of their developed radio-frequency CNT based RF-transistors even further by optimizing their composition and structure.
More information about this research can be found at http://dx.doi.org/10.1038/s41928-021-00594-w.