Industrial Electronics

From silicon- to nanoribbon-based transistors

16 December 2022
Structure of graphene nanoribbon. Source: Purdue University

When Moore's law was established in 1965, the world of electronics underwent a dramatic change. It stated that the number of transistors on a chip will double roughly every two years. Since then, the channel length has shrunk in tandem with the increase in chip capacity. Scaling down silicon (Si) transistors in an exponential pattern has improved device performance and density but has also increased the density of logic circuits. Despite the fact that the performance has improved by 40% and the memory capacity has quadrupled, additional criteria must be considered. This is because of the scaling-down of metal oxide semiconductor field-effect transistors (MOSFETs), which has made future fundamental constraints apparent.

Why is a there a need for nanoribbon-based transistors?

Dopant fluctuations, short-channel effect and mobility degradation are making it difficult keep pace with Moore's idea in Si-based tech below 7 nm. This is why carbon-based technology's potential is being examined for possible successors to Si's complementary metal-oxide semiconductors (CMOS). Device dimensions are shrinking as a result of current CMOS-based technologies' limitations. The gate leakage current is addressed by reducing the gate oxide layer to less than 3 nm, leading to declining dielectric performance. Despite this remarkable effect, the performance appears to be reducing due to short-channel effects such as hot carrier effects, direct tunneling gate leakage, drain-induced barrier lowering and gate-induced drain leakage.

Like other physical constraints on Si CMOS, channel doping is required to reduce short-channel effects. However, due to impurity dispersion and the transverse electric field, channel doping reduces band-to-band tunneling and carrier mobility. As a result of the depletion charge channel's capacitive impact, the on-state current amount is lowered. It also causes resistance and lowers the drain current because of the activation of dopants. The growing costs of production and testing are also putting pressure on the economy. With the inability to produce devices below the light wavelength, Si CMOS is proving deficient.

International semiconductor technology roadmaps indicate that CMOS technology is nearing the quantum mechanical physics limit. Due to the inability of conventional Si to scale any farther than the 7-nm range, researchers are looking at new technologies to replace it. And that is where the nanoribbon-based transistors come into play.

What are nanoribbon-based transistors?

There has been a lot of investigation into graphene's remarkable features, particularly its tremendous electron mobility. The edges of graphene are passivized by hydrogen, carboxyl, oxygen or hydroxyl groups, which function as absorbents, to classify it as monolayer, bilayer or multilayer. Moreover, graphene has received a lot of interest because of its high carrier mobility, ballistic transport due to its long mean free path, and great resilience in the electronics domain.

Graphene nanoribbons and carbon nanotubes have also been the subject of substantial research into electron transport. There is a growing interest within the electrical device community in graphene nanoribbons, as they show electronic band gaps due to quantum captivity of charge carriers, unlike graphene, which has two dimensions. Based on chirality, Graphene nanoribbons may be classified as either armchair or zigzag (depending upon the conductivity and energy gap).

In field effect transistors, graphene nanoribbons can be employed as a channel material, allowing for correct switching behavior. Because of the thin structure of graphene and increased controllability, flexible electronics are looking to graphene as a basis material. For the first time, researchers have employed graphene-based nanoribbons to create transistors with a sub-10 nm fabrication process. These transistors are constructed by embedding graphene nanoribbons between a source and a drain. A great conductor by nature, 2D graphene is transformed into a great massless semiconductor when 1D graphene nanoribbons are extracted. However, this is a difficult process.

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Challenges

In comparison to conventional Si-based CMOS, graphene nanoribbon field-effect transistors have been found to have striking properties, such as a higher on-state and off-state current ratio and a smaller subthreshold swing. These structures are capable of alleviating some of the current issues. However, some of these designs have been questioned because of features like graphene's zero bandgap and leakage and field emission that dominate at higher voltages, even though the advantages outweigh these drawbacks.

Conclusion

Scientists are investigating carbon-based nanodevices that have capabilities similar to those of Si but are more mobile and practical at the nanoscale level as a way to deal with Moore's law and the limits of production procedures specifically developed for Si-based CMOS. Carbon nanotubes and graphene nanoribbon-based transistors are being considered as possible alternatives to Si, which is currently used in most electronic devices. Graphene is more popular now due to its high resilience, high carrier mobility and high ballistic transport. Moreover, research and development in this area is needed, as well as models for the graphene nanoribbon-based transistors structures that can be used for both digital and analog applications.

To contact the author of this article, email GlobalSpecEditors@globalspec.com


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