TECH
Dog-bone design helps 2D nanoribbon transistors stay fast and efficient as widths shrink
Transistors, small semiconductor-based switches that control the flow of electricity, are central components of all electronic devices, from computers to smartphones, wearables, sensors and smart appliances. Over the past decades, electronics engineers have been continuously working to boost the speed and performance of transistors while also reducing their size.
A promising approach for miniaturizing transistors entails the use of two-dimensional (2D) semiconductors, materials that are only one or a few atoms thick. Despite their potential, most high-performing 2D transistors have so far been demonstrated using relatively wide channels, and it has remained unclear whether their performance can be preserved when the channels are made much narrower.
Researchers at Stanford University recently developed new compact transistors based on narrow strips of monolayer 2D semiconducting materials known as nanoribbons. These transistors, introduced in a paper published in Nature Nanotechnology, were found to perform remarkably well despite their small size, outperforming previously developed nanoribbon transistors based on the same 2D materials.
"We wanted to reduce 2D transistors in all dimensions, including width," Eric Pop, senior author of the paper, told Tecplor. "Using a monolayer 2D semiconductor, the channel is automatically sub-nanometer thin, but to be technologically relevant, such transistors should also be very small in both length and width."
The team's recent study specifically focused on reliably reducing the transistors' width without significantly affecting their performance.
"Most academic works have looked at the 2D channel's thickness and length, which motivated us to systematically study width scaling in these materials and their devices," said Tara Peña, co-first author of the paper.
To prevent nanoribbon delamination (i.e., peeling from the surface during fabrication), the researchers employed a new approach, patterning 2D semiconductors into a dog bone-like shape. Metal contacts were integrated on the wider regions of this bone-like pattern, acting as anchors.
"This approach allowed us to study many nanoribbon channels as the narrow part of the dog bone, for several 2D semiconductors," said Pop. "In future industry use cases, the anchoring of nanoribbons will need to be achieved in a more compact way."
Using their approach, the researchers fabricated nanoribbon transistors based on three different monolayer 2D semiconductors, namely MoS2, WS2 and WSe2. Electrical measurements showed that the narrow nanoribbon channels retained good transistor behavior across all three materials.
"Importantly, the nanoribbons all behaved well with our nanofabrication approach at dimensions down to about 25 nanometers, including both n- (MoS2, WS2) and p-type behavior (WSe2)," explained Pop.
"This means that the edges are not fundamentally limiting the performance of these materials, and the edges could be further improved. The WS2 transistors were also able to carry about one hundred times higher current density than previous demonstrations, partly due to our improved contacts."
When they tested their newly developed transistors, Pop and his colleagues were surprised to discover that they did not show higher off-state leakage than wider transistors based on the same 2D materials. This suggests that despite their reduced width, the edges of the nanoribbons did not cause excess leakage, which is important for low-power operation.
"Another important part of our approach to reach the narrowest widths was how we etched the transistor channels," said Anton Persson, co-first author of the paper. "Instead of etching the channel in one step, we used two separate etching steps, which etched the channel from opposite sides. This allowed us to form narrower channels than with the conventional one-step approach."
The transistors developed by the researchers achieved good on-state currents of 560 µA/µm for n-type MoS2, 420 µA/µm for n-type WS2 and 130 µA/µm for p-type WSe2. Notably, all three transistors performed better than most other nanoribbon 2D transistors introduced in the past.
"We found that nothing dramatic happens when the transistors become very narrow," said Persson. "We were concerned that the etched-out edges of these semiconductors would cause problems, but the devices still behaved well or at least similarly to their wider counterparts. This suggests that these monolayer 2D semiconductor channels are relatively robust when scaled down in width."
Future research directions...The design and fabrication strategies introduced by this research team could soon be refined further and used to create other electronic components based on 2D semiconductors. This study demonstrated the potential of these strategies for realizing extremely small devices.
"The 'dog-bone' design and multi-step etching approaches both helped with adhesion and achieving narrower widths," said Peña.
"We also believe reducing electron beam dose and polymer contamination during the fabrication process allowed us to obtain 'cleaner' edges for our 2D nanoribbons. We hope our work will inspire other groups (and industry) to think carefully about how to limit sources of disorder that ultimately impact 2D device performance."
Pop and his colleagues are now planning further studies aimed at evaluating their proposed design and patterning approach. They will also try to realize nanoribbons that can operate at lower voltages, with improved edges and smaller contacts.
"For example, here we showed pretty good behavior is possible at 1 V drain-to-source voltage," explained Pop. "It will be important to achieve good behavior at 0.5 V on both the drain and gate, in order for these to be considered viable alternatives to silicon nanosheet transistors."
As part of their next studies, the researchers are curious to determine how far their downscaling approach can go before the performance of transistors starts declining.
"We expect that the devices will eventually start to degrade as the channels become even narrower, but we do not yet know at what width that happens," said Persson. "Understanding what happens below 10 nanometers in width will eventually be important if 2D semiconductors are to be compared seriously with future silicon nanosheets."
"Understanding how various strains and defects impact these ultra-scaled 2D nanoribbons will also be critical, which will require sophisticated materials characterization approaches," added Peña.
2D nanoribbons are ultra-narrow, strip-like structures of two-dimensional (2D) materials (like graphene, transition-metal dichalcogenides, or boron nitride) with widths typically scaled to under 50 nanometers. By restricting 2D sheets into 1D-like ribbons, researchers unlock tunable bandgaps, high charge mobility, and superior quantum edge effects.
Why 2D nanoribbons matter:
-Bandgap engineering: Unlike pristine 2D sheets (such as zero-bandgap graphene), nanoribbons exhibit tunable electronic bandgaps, which are strictly dictated by their precise width and edge orientation.
-Enhanced performance: Nanoribbon architectures offer highly exposed active edges that facilitate rapid electron transport and improved charge mobility, making them ideal for scaling down semiconductor logic devices.
-Quantum confinement: The extremely narrow channels lead to quantum effects, giving rise to unique magnetic behaviors, spin-filtering capabilities, and potential applications in quantum computing.
Key applications:
-Next-generation nanoelectronics: Nanoribbons are highly promising for the creation of ultra-compact Field Effect Transistors (FETs). For instance, researchers have developed high-speed, 35 nm channel nanoribbon transistors using monolayer 2D semiconductors to maintain performance despite shrinking dimensions.
-Energy storage & catalysis: The high surface-area-to-volume ratio and edge activity allow 2D nanoribbons (like those derived from unzipped carbon nanotubes) to act as excellent catalysts for hydrogen evolution, oxygen reduction, and energy storage.
-Optoelectronics: Their tunable optical and electrical properties make them prime candidates for advanced solar cells, sensors, and photodetectors.
Common types:
-Graphene nanoribbons (GNRs): The most well-studied type, often created using bottom-up molecular polymerization or unzipping of carbon nanotubes. They are prized for ballistic electron transport.
-Transition-metal dichalcogenide (TMD) nanoribbons: Materials like MoS₂ are naturally semiconducting and thin, making them optimal for extreme miniaturization in modern computing without the leakage currents found in traditional silicon.
-Boron nitride nanoribbons (BNNRs): Structurally similar to graphene but with a large bandgap, often used as an insulating dielectric layer or combined with graphene in heterostructures.
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