“The performance gains we have demonstrated were predicted over a decade ago, based on the performance of an isolated single CNT transistor,” explains team member Gerald Brady, “but this is the first time that a conductance this high has been realized in a field-effect transistor (FET) based on an array of CNTs.”

Carbon nanotubes (CNTs) are sheets of carbon just one atom thick that have been rolled up into a tube with a diameter of about 1 nm. Semiconducting nanotubes can be used to build transistors, for example, and could even replace silicon in a host of future nanoelectronics devices because they are tiny, but can still carry huge amounts of current. The tubes also boast high switching speeds and high on-off current ratios.

Materials science challenges

However, the problem is that although individual CNTs have almost ballistic conductance, the same is not the case when they are made into arrays. This is because it is very difficult to form highly conductive contacts in bundles of CNTs, something that results in both a decreased conductance and current on/off ratios in FETs made from the materials. What is more, an ideal CNT array consists of purely semiconducting-type CNTS with a uniform band gap, but when CNTs are synthesized, a mix of both semiconducting and metallic tubes is produced. The presence of even a single metallic CNT impurity can short-circuit an FET, lowering the on-off ratio by several orders of magnitude.

Now, a team led by Michael Arnold at the University of Wisconsin-Madison has succeeded in making quasi-ballistic CNT arrays with a density of 50 CNTs/μm2, with each CNT in the array having a conductance as high as 0.46 Go. To compare, the maximum theoretical conductance limit for a pristine CNT is 2 Go.

CNT array current density is nearly twice that of silicon's

The researchers began by using a selective polymer wrapper to purify their CNTs and then deposited these nanostructures into aligned arrays on substrates using a solution-based processes called Floating Evaporative Self-Assembly. This process organizes the tubes into high-density arrays with a uniform inter-CNT spacing. “Finally, we surface treat the CNT arrays to remove solution processing-based impurities from the films before fabricating them into devices,” explains Brady.

“When the transistors are turned on to the conductive state (meaning that current is able to pass through the CNT channel), the amount of current travelling through each CNT in the array approaches the fundamental quantum limit,” he tells nanotechweb.org. “Since the CNTs conduct in parallel, and the packing density and conductance per tube are very high, the overall current density is very high too – at nearly twice that of silicon’s. The result is that these CNT array FETs have a conductance that is seven times higher than any previous reported CNT array FET.

“The high semiconducting purity of the CNTs also allows the FETs to be turned off completely, which is critical for real electronics applications that must maintain low power consumption in the off-state,” he adds.

Logic applications, wireless communications and thin-film technologies

Such high current-density nanoscale (roughly 100 nm) CNT transistors could be employed in logic applications, he says. “The implication here is that by replacing silicon with a CNT channel, it should be possible for us to make either a higher performing device or one that works at lower power.”

The high current density and the fact that the CNTs are pure means that they could be used in radio-frequency amplifiers for wireless communications and thin-film technologies such as flat-panel displays that require high charge-carrier mobilities and transparent materials, which CNTs are. And that is not all: the procedure employed by the Wisconsin team to create the CNT arrays are compatible with most device processing and fabrication schemes and could be adapted to integrate the CNT materials in a variety of application areas.

The researchers, reporting their work in Science Advances DOI: 10.1126/sciadv.1601240, say that they are now working on further increasing the current density of their CNT-array FETs. “We also plan to reduce the variability between the devices we make and adapt a number of features developed for single CNT transistors to further demonstrate their commercial value.”

Brady acknowledges funding from the National Science Foundation, the US Army Research Office and the Air Force.