CTO Chris Rutherglen from Carbonics Inc. Discusses the Use of Carbon Nanotubes in Electronics

Since their discovery a few decades ago, materials made of carbon nanoparticles, known as carbon-based nanomaterials, have attracted great interest from the electronics industry and research community, as they could enable better performance and lower power consumption in a vast array of devices.

A few examples of these materials are carbon nanotubes (CNTs), fullerenes, carbon nanofibers, carbon-onions, and carbon black. Depending on how they are engineered and synthesised, these materials can have a variety of useful applications in industrial, medical, agricultural, or technical settings. CNTs, for instance, have been found to exhibit a number of desirable properties, including remarkably high electrical and thermal conductivities. 

Carbonics Inc, a California-based company that specialises in the development of semiconductors and nanoelectronics, has been exploring the potential of carbon nanomaterials for several years now.

Carbonics was founded in 2014 with the key mission of revolutionizing the electronics industry, by promoting the use of carbon nanomaterials, which are far more abundant than other materials currently used to build electronics. The company conducts extensive studies, while also leveraging research carried out at UCLA, KACST, and USC. 

The work of Carbonics is ultimately aimed at integrating carbon nanomaterials into radio frequency (RF) components, in order to reduce the battery demands and efficiency of electronic devices.

Chris Rutherglen, CTO at Carbonics Inc, has been exploring the potential of carbon-based nanomaterials for several years now. In this interview with Electronics Points, he explains what drew him to this particular field of research, highlighting some of the most recent advancements in RF technology and CNT development.  

Chris Rutherglen, Chief Technology Office at Carbonics Inc. 

Ingrid Fadelli: Please tell us a little about your engineering background and what led you to the position of Chief Technology Officer at Carbonics, Inc. 

Chris Rutherglen: After high school, I became increasingly interested in physics and anything related to RF (radio frequency). Since I sought to understand how things work at their most basic level, as an undergraduate I studied physics at the California Institute of Technology and then went more practical by pursuing my graduate studies in electrical engineering at the University of California, Irvine.

At the time, carbon nanotubes were one of the new hot research topics and I ended up with Professor Peter Burke’s research group that, among other things, focused on carbon nanotubes for RF applications. My work at Carbonics is essentially a continuation of the work that I completed during my graduate school days, over 10 years ago.

IF: Your academic background suggests that you’ve specialized in RF technology early on in your career. What are your reasons for focusing on that subject specifically? 

CR: Since I was very young, radio communications always fascinated me. Back then, when I was around 7 years old, my brother and I used to play games and used a two-way radio to talk to each other, even though we were in some cases miles apart. Mind you, this was in the late 1980s, long before mobile phones went mainstream.

Back then I would puzzle over how my voice could travel such a seemingly large distance, even though there was nothing physically connecting these two radios. At least in the case of telephones (landlines telephones in today’s terminology), I was able to rationalize it, because there were physical wires connecting both ends.

With radio technology, however, there was nothing but some invisible pathway connecting two devices. It was this particular aspect that fascinated me and sparked the curiosity that seems to have endured ever since.

IF: What are the memorable aspects and takeaways from any leadership position you’ve had or teams that you’ve managed? How does a managing position differ from your experiences working as a team member?

CR: At Carbonics, we have a very small team of four scientists & engineers who all hold a PhD. Because of this, management duties are relatively low compared to how they may be in other settings, which allows for most of my time to be spent working alongside the team in the lab. Since they are all very capable PhD-level scientists and engineers, once given a general direction, they figure out a solution with minimal supervision. Having such a team makes managerial tasks relatively easy.

IF: Are there any specific areas of R&D that you would like to explore more in the future?

CR: One of the unique characteristics of carbon nanotubes—which sets them apart from standard semiconductors such as silicon—is that they can be applied via a surface coating method, as opposed to being intrinsic to the substrate itself. For example, while a silicon transistor will need a silicon substrate, for carbon nanotubes the substrate is not a constraint since the semiconducting material is applied on the surface using a room-temperature process.

This allows carbon nanotubes to be placed on a wide variety of substrates. In addition to making it easily integratable with silicon CMOS technology, this characteristic means that carbon nanotubes can be applied to flexible substrates that can contour to an arbitrary form; or that they can be used to create multiple layers of transistors, one layer on top of another, forming a three-dimensional stack.

This would be very difficult to do with standard incumbent technologies, but this unique feature makes it possible with CNT transistor technology. In fact, a DARPA-funded project is currently exploring this application, using a mix of carbon nanotube transistors and CMOS.

Densely-aligned carbon nanotubes are deposited on a wafer using a simple surface coating method, followed by pattern and metallization of the electrodes using standard, scalable lithography tools. Here we see results of densely-aligned carbon nanotube transistors from the wafer level all the way down to the device level. 
 

IF: What important considerations come to mind for research and development efforts in RF, CNT, and other technologies you’re working on?

CR: Advancing new technology, such as carbon nanotube transistors for RF, can be a long and slow process. It has now been over ten years since the first efforts in this area and we will most likely need to wait another ten years of further development before commercial products become available. 

This long development period is mainly a function of how mature and well-established the incumbent technologies are in meeting the needs of current applications. For new transistor technology to take hold, it needs to offer an advantage in performance or functionality that the incumbents can not meet. This is why significant time and money are needed to bring the technology to that level.

IF: You have been involved with numerous research studies related to CNT and RF research. Can you share the developments or findings that defined those studies?

CR: Late last year we published results in Nature Electronics showing, for the first time, that carbon nanotube field effect transistors can compete with incumbent RF semiconductor technologies in some key performance metrics. Furthermore, by making some simple extrapolations of widely repeatable single carbon nanotube devices results, we are confident that with further refinement the technology will ultimately surpass the incumbent technologies, GaAs and RF-CMOS, on nearly all metrics. 

The key is to take that single carbon nanotubes device performance and translate it into a device with thousands of carbon nanotubes all operating in parallel. That is what we have been working on and continue to work on at Carbonics.

IF: Could you tell us a little bit about your work in creating CMOS compatible microwave amplifiers and explain how it could impact the engineering field at large?

CR: As this new decade begins, we will be hearing a lot more about 5G, the fifth-generation wireless technology standard that is currently being rolled out.  One of the two generalized frequency bands that will be used for 5G is the new millimeter-wave (mmWave) band, which allows for very high-speed data transfers due to the large bandwidths available. On a device and circuit level, however, things become a lot more complicated at these higher frequencies.

Specifically, on-circuit signal loss becomes a significant burden to performance, due to the use of wire bonds and long circuit traces, typically associated with the multichip module packaging methods used in the sub-6GHz bands. For mmWave circuits, it is almost a necessity to have all of the RF front-end, which is the circuit elements between the modem and the antenna, all on a common substrate or one chip.

One can achieve such an integrated circuit using RF-CMOS technology, but at the cost of lower performance compared to the other available technologies, for instance, GaAs, which has high performance but cannot be integrated onto a silicon substrate. Carbon nanotubes transistor technology could potentially bridge this gap since it uses a simple surface coating method and can easily be integrated or placed on the same silicon substrate surface as digital CMOS control circuitry.   

IF: Please share the motivations you have for conducting research to advance technologies in your field. What role or level of importance do you think research has for engineers, regardless of specialties?

CR: For engineers and society in general, most discoveries and innovations originate through research. For those active in a particular field of research, the excitement of discovery and trailblazing a path to new technology can be quite motivating. That said, one also has to keep in mind that it can be a long process to blaze that path and dead-ends along the way are common. Yet that is just part of the research process.

IF: What future developments or achievements are you planning for or anticipating for RF or CNT technologies?

CR: As CNT technology continues to improve and the board-based superior performance can be demonstrated, the effort will then be placed into transitioning the fabrication process to a foundry setting. Subsequently, further refinements and scaling of the technology will be the main focus.

IF: What are your thoughts on furthering CNT technology specifically? What potential does it have for certain applications or products for the engineering industry as a whole?

CR: In addition to what has already been mentioned regarding the integration with CMOS for mmWave applications, another potentially game-changing feature of CNT transistor technology is the potential for extraordinarily high linearity. By this, I mean that the input signal can be reproduced at an amplified power-level, all without the introduction of distortion. As one knows from an audio system, if the input signal is turned up too high or someone talks too loud into a microphone, what comes out of the speakers will be cringe-worthy noise, as opposed to a soothing melody.

A similar thing happens in radio frequency amplifiers. If the input power is too high and the amplifier has poor linearity, the output signal will be a badly distorted amplified copy of the input. Due to a unique property of carbon nanotubes, specifically that they are one-dimensional materials, some predict that they would possess extraordinarily high linearity and thus be able to reproduce, without distortion, the signal even with relatively high input powers.

If this can be proven, it would open the door to a wide range of applications from radar to the use of higher-spectral efficient modulation schemes, all at lower power levels. This would help to reduce the size and weight of batteries.

Since carbon nanotube technology can be applied to a wide variety of substrates and is expected to have superior RF performance, Rutherglen and his colleagues at Carbonics Inc. think that it could fill a niche in the set of RF semiconductor technology offerings currently available. The incumbent technology RF-CMOS has the capability for high levels of integration, but its performance is not as good. At the other extreme is GaAs pHEMT technology, which has a high-performance level, but since such devices are fabricated on GaAs substrates, they cannot be easily integrated with silicon CMOS. 

IF: What are some major technologies or products that Carbonics is working on that you’d like to inform engineers of? What makes them significant and what applications or industries are they designed for?

CR: Ultimately, the target for RF and millimeter-wave circuits is the large and growing consumer products market. However, the natural progression is to first target the technology’s early adopters, who tend to look for high-performance, are less cost-sensitive, and can work with the technology as it matures.

This tends to entail the development of products for aerospace, military, and industrial applications. An example of this would be an advanced radar. Gradually, fabrication yields and volumes go up which drives cost down and the price-sensitive consumer products market begins to open up for a wide variety of applications, such as mmWave products.

IF: What are your predictions and hopes for future development in the engineering industry over the next decade or so?

CR: As we have seen over the past decade with smartphones, smartwatches, smart light bulbs, etc., wireless data links are becoming the standard and are now much more ubiquitous. 

With the onset of the 5G mobile communications and its impact on internet-of-things (IoT) applications, the RF semiconductor industry should continue to see strong growth over the next ten years. That said, the semiconductor industry remains a cyclical business, so the growth I am talking about would be overlaid with the normal vicissitudes of the business cycle.

In years to come, CNTs and other carbon nanomaterials could play an increasingly crucial role in the development of electronic components. Chris Rutherglen and his colleagues at Carbonics will continue to investigate ways in which these materials could enhance the performance and efficiency of a wide variety of devices.

Their findings and the components they develop could ultimately contribute to the production of future generations of electronics. To can learn more about Carbonics or read about their latest R&D effort, visit Carbonics Inc's website.