Carbon Nanotube Tech Exceed 100 GHz for RF Applications

Article By : Nitin Dahad

Carbon nanotubes are becoming a serious competitor to silicon in almost all areas of microelectronics.

Carbonics, Inc., has demonstrated a wafer-scalable approach for producing an array of aligned carbon nanotube (CNT) FETs with performance exceeding 100 GHz and linearity of 10dB.

The firm, based on Los Angeles, is a spinout from the Center of Excellence for Green Nanotechnologies at UCLA and King Abdulaziz City for Science and Technology (KACST).

Carbonics said this is the first time carbon nanotube technology has achieved speeds exceeding 100GHz in radio frequency (RF) applications. Reported in a paper published this month, “Wafer-scalable, aligned carbon nanotube transistors operating at frequencies of over 100 GHz,” it added the milestone ‘eclipses’ the performance — and efficiency — of traditional RF CMOS technology ubiquitous in modern consumer electronics. That includes cell phones, where the superior performance could boost 5G wireless and other mmWave technologies.

A separate commentary in the journal called the research “a remarkable technology milestone,” and concluded that we are now, finally, close to a tipping point in which nanotubes become a serious competitor to silicon in almost all areas of microelectronics.

For nearly two decades, researchers have theorized that carbon nanotubes would be well suited as a high-frequency transistor technology due to its unique one-dimensional electron transport characteristics. The engineering challenge has been to assemble the high purity semiconducting nanotubes into densely aligned arrays and create a working device out of the nanomaterial.

As reported in the paper, Carbonics, a venture backed startup, has successfully overcome this challenge and for the first time demonstrated device performance exceeding RF CMOS in key RF metrics. Projections based on scaling single carbon nanotube device metrics suggest the technology could ultimately far exceed the top-tier incumbent RF technology, GaAs.

 Carbonics graphic

Carbonics’ carbon nanotube technology made up of semiconducting aligned carbon nanotubes (the transparent lattice tubes at the bottom of this image) within a transistor device structure. The quasi ballistic transport within the carbon nanotubes acts as a superhighway for electrons, resulting in a superfast and linear RF technology.

Wireless device technology operating in the millimeter wave regime (30 to 300 GHz) increasingly needs to offer both high performance and a high level of integration with complementary metal–oxide–semiconductor (CMOS) technology. III–V technologies, such as GaAs pseudomorphic high-electronmobility transistors (pHEMTs), have been the standard for high linearity, low-noise applications in the microwave and millimeter bands for decades.

However, their use of GaAs substrates makes them incompatible with Si CMOS integration, which is a major limitation that has led to the loss of market share to RF CMOS, despite the fact that GaAs offers superior performance.

With their highly linear signal amplification and compatibility with CMOS, aligned carbon nanotubes (aCNT-FETs) naturally become interesting as an alternative to III–V technologies.

Carbonics’ technology relies on a simple, room-temperature surface-coating method to apply nanotubes to a substrate. This decouples the semiconductor material from a specific type of bulk substrate and allows high-level integration with CMOS for system-on-chip-level complexity, which has been demonstrated recently for a mixed CNT–CMOS digital circuit application. Unlike the incumbent technologies, such as pHEMTs, which create charge confinement within a bulk material via a heterostructure-derived two-dimensional electron gas layer, the charges in CNTs are naturally confined in one dimension, which leads to desirable transport characteristics.

The firm said quasiballistic transport has been observed for tube lengths of 300 nm, which is a direct consequence of its reduced scattering degrees of freedom. Empirically observed current density and transconductance values of up to 25 μA and 20–40 μS, respectively, have been reported for single-tube measurements, which on scaling into dense arrays would exceed those of the incumbent technologies.

Although these CNT characteristics have been known on an individual tube basis, to make a useful amplifier it is necessary to produce an array of thousands of aligned semiconducting CNTs operating in parallel. The two primary engineering challenges for radio-frequency CNT-FETs — enriching as-produced CNTs to a high semiconducting fraction (>99.9%) and assembling them into densely aligned arrays (up to 70 tubes μm−1) — have been overcome. The challenge now turns to device-level process improvements that will harness the fundamental material advantages of CNTs.

The paper reports the wafer-scalable fabrication of an array of aligned carbon nanotube field-effect transistors operating at gigahertz frequencies. The devices have gate lengths of 110 nm and are capable, in distinct devices, of an extrinsic cutoff frequency and maximum frequency of oscillation of over 100 GHz, which surpasses the 90 GHz cutoff frequency of radio-frequency CMOS devices with gate lengths of 100 nm and is close to the performance of GaAs technology.

Carbonics said its devices also offer good linearity, with distinct devices capable of a peak output third-order intercept point of 26.5 dB when normalized to the 1 dB compression power, and 10.4 dB when normalized to d.c. power.

 Carbonics fabrication diagrams

Aligned CNT-FET devices fabricated with a wafer-scalable process. a) aCNT-FET T-gate devices fabricated on 100 mm quartz wafers and patterned with a 7 × 7 array of dies each containing 46 available devices plus 2 calibration shorts; b) optical micrograph; c) coplanar waveguide structure of a single aCNT-FET; d) SEM image of one edge of aCNT-FET; e) SEM image of a T-gate centred in the channel; f) Depiction of aCNT-FET device cross-section. (Source: Nature Electronics / Carbonics)

The company’s deposition technology, called Zebra, enables carbon nanotubes to be densely aligned and deposited onto a variety of chip substrates including silicon, silicon-on-insulator, quartz and flexible materials. This allows the technology to be directly integrated with traditional CMOS digital logic circuits and overcomes the typical problem of heterogeneous integration.

“This milestone shows that carbon nanotubes, long thought to be a promising communications chip technology, can deliver,” said Dr. Joe Qiu, program manager at the Army Research Office, who reviewed the research. “The next step is scaling this technology, proving that it can work in high-volume manufacturing. Ultimately, this technology could help the army meet its needs in communications, radar, electronic warfare and other sensing applications.”

Carbonics’ CEO Kos Galatsis said the accomplishment means the timing is ripe to leverage its CMOS-compatible technology for 5G and mmWave defense communication markets. “We are now engaged in licensing and technology transfer partnerships with industry participants, while we continue to advance this disruptive RF technology,” he added. Carbonics believes that its Zebra technology will likely first be adopted in military applications before being used more broadly.

In 2014, Carbonics was spun-out from the joint center of UCLA-USC and KACST called the Center of Excellence for Green Nanotechnologies and academic funding support from SRC, DARPA and the U.S. Air Force. The work published in Nature Electronics was funded by the US Army contract No. W911NF19P002 and by KACST.

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