Power/Performance Bits: Sept. 6

Carbon nanotube transistors outperform silicon; flexible two-terminal tunneling memory; analog DNA circuit.

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Carbon nanotube transistors outperform silicon

University of Wisconsin-Madison materials engineers created carbon nanotube transistors that outperform silicon transistors, improving the current 1.9 times. The new transistors are particularly promising for wireless communications technologies that require a lot of current flowing across a relatively small area.

“This achievement has been a dream of nanotechnology for the last 20 years,” said Michael Arnold, a UW-Madison professor of materials science and engineering. “Making carbon nanotube transistors that are better than silicon transistors is a big milestone. This breakthrough in carbon nanotube transistor performance is a critical advance toward exploiting carbon nanotubes in logic, high-speed communications, and other semiconductor electronics technologies.”

Carbon nanotube transistors should be able to perform five times faster or use five times less energy than silicon transistors, according to extrapolations from single nanotube measurements. The nanotube’s ultra-small dimension makes it possible to rapidly change a current signal traveling across it.

The UW–Madison engineers used a solution process to deposit aligned arrays of carbon nanotubes onto 1 inch by 1 inch substrates. The researchers used their scalable and rapid deposition process to coat the entire surface of this substrate with aligned carbon nanotubes in less than 5 minutes. (Source: Stephanie Precourt)

The UW–Madison engineers used a solution process to deposit aligned arrays of carbon nanotubes onto 1 inch by 1 inch substrates. The researchers used their scalable and rapid deposition process to coat the entire surface of this substrate with aligned carbon nanotubes in less than 5 minutes. (Source: Stephanie Precourt)

Creating the nanotubes presented a number of challenges.

Isolating the semiconducting nanotubes was crucial, because metallic nanotube impurities act like copper wires and disrupt their semiconducting properties. The team was able to create a solution of ultra-high-purity by using polymers to sort out semiconducting carbon nanotubes.

They also pioneered a technique called “floating evaporative self-assembly” to ensure good alignment of the nanotubes and baked the nanotube arrays in a vacuum oven to remove the insulating layer created by the polymer sorting and improve electrical contact.

The researchers benchmarked their carbon nanotube transistor against a silicon transistor of the same size, geometry and leakage current.

They are continuing to work on adapting their device to match the geometry used in silicon transistors. Work is also underway to develop high-performance radio frequency amplifiers. While the researchers have scaled their alignment and deposition process to 1 inch by 1 inch wafers, they’re working on scaling the process up for commercial production.

TRAM: flexible two-terminal tunneling memory

Researchers from the Institute for Basic Science (IBS) in Korea and Sungkyunkwan University devised a new flexible memory device inspired by the neuron connections of the human brain.

IBS scientists constructed the memory, called two-terminal tunneling random access memory (TRAM), with two electrodes resembling the two communicating neurons of the synapse. Two-terminal memories have an advantage in that they do not need a thick and rigid oxide layer, unlike three-terminal flash memory. “Flash memory is still more reliable and has better performance, but TRAM is more flexible and can be scalable,” said Professor Yu Woo Jong.

TRAM is made up of a stack of one-atom-thick or a few atom-thick 2D crystal layers: One layer of the semiconductor molybdenum disulfide (MoS2) with two electrodes (drain and source), an insulating layer of hexagonal boron nitride (h-BN) and a graphene layer. TRAM stores data by keeping electrons on its graphene layer. By applying different voltages between the electrodes, electrons flow from the drain to the graphene layer tunneling through the insulating h-BN layer. The graphene layer becomes negatively charged and memory is written and stored and vice versa, when positive charges are introduced in the graphene layer, memory is erased.

The appropriate thickness of the h-BN isolating layers allows electrons to tunnel and reach the graphene layer without leakages. H-BN layers of different thicknesses were tested and a thickness of 7.5 nanometers was found to be the most appropriate. (Source: IBS)

The appropriate thickness of the h-BN isolating layers allows electrons to tunnel and reach the graphene layer without leakages. H-BN layers of different thicknesses were tested and a thickness of 7.5 nanometers was found to be the most appropriate. (Source: IBS)

IBS scientists carefully selected the thickness of the insulating h-BN layer, as they found that a thickness of 7.5 nanometers allows the electrons to tunnel from the drain electrode to the graphene layer without leakages and without losing flexibility.

Flexibility and stretchability were two key features of TRAM. When TRAM was fabricated on flexible plastic (PET) and stretchable silicone materials (PDMS), it could be strained up to 0.5% and 20%, respectively. In the future, TRAM can be useful to save data from flexible or wearable smartphones, eye cameras, smart surgical gloves, and body-attachable biomedical devices.

According to the researchers, TRAM showed better performance than other two-terminal memories, phase-change random-access memory (PRAM) and resistive random-access memory (RRAM).

Analog DNA circuit

A Duke University team created strands of synthetic DNA that, when mixed together in a test tube in the right concentrations, form an analog circuit that can add, subtract and multiply as they form and break bonds.

Previously, DNA-based circuits have been designed to solve problems ranging from calculating square roots to playing tic-tac-toe, but most of these are digital.

However, commercial applications of DNA circuits are still a long way off, according to Duke professor John Reif. For one, the test tube calculations are slow. It can take hours to get an answer. But DNA circuits can be tiny, and work in wet environments, which might make them useful for computing inside the bloodstream or the cell.

The technology takes advantage of DNA’s natural ability to zip and unzip to perform computations. Just like Velcro and magnets have complementary hooks or poles, the nucleotide bases of DNA pair up and bind in a predictable way.

The researchers first create short pieces of synthetic DNA, some single-stranded and some double-stranded with single-stranded ends, and mix them in a test tube. When a single strand encounters a perfect match at the end of one of the partially double-stranded ones, it latches on and binds, displacing the previously bound strand and causing it to detach. The newly released strand can in turn pair up with other complementary DNA molecules downstream in the circuit, creating a domino effect.

Problems are solved by measuring the concentrations of specific outgoing strands as the reaction reaches equilibrium.

To see how their circuit would perform over time as the reactions proceeded, researchers simulated the reactions over a range of input concentrations and have also been testing the circuit experimentally in the lab.

Besides addition, subtraction and multiplication, the researchers are designing more sophisticated analog DNA circuits that can do a wider range of calculations, such as logarithms and exponentials. The lab is also beginning to work on DNA-based devices that could detect molecular signatures of particular types of cancer cells, and release substances that spur the immune system to fight back.