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Two-Dimensional Materials Tackle the Diode

The p-n heterojunction diode, one of the most common building blocks in electronics, can be made from a 2-D material

2 min read
Two-Dimensional Materials Tackle the Diode
Image: Northwestern University

While graphene by itself has been generating enormous interest in both the research community and outside of it, what many are still missing is that we are in the midst of a two-dimensional material explosion that goes beyond just graphene.

Molybdenum disulfide (MoS2) is beginning to take center stage right behind graphene in the cast of 2-D materials that includes silicene (a single layer of silicon) and boron nitride. Nearly three years ago, MoS2 was revealed as a possible 2-D replacement for three-dimensional silicon in transistors.

Even though MoS2 had an advantage over graphene in that it has an inherent band gap, it was really first imagined as a complement to graphene in applications such as optoelectronics and energy harvesting, where thin, transparent semiconductors are required.

Now researchers at Northwestern have gone back to MoS2's complementary role and combined it with carbon nanotubes to create p-n heterojunction diode. The p-n junction forms the backbone of devices such as solar cells, light-emitting diodes, photodetectors, and lasers.

“The p-n junction diode is among the most ubiquitous components of modern electronics,” said Mark Hersam, director of the Northwestern University Materials Research Center, in a press release. “By creating this device using atomically thin materials, we not only realize the benefits of conventional diodes but also achieve the ability to electronically tune and customize the device characteristics. We anticipate that this work will enable new types of electronic functionality and could be applied to the growing number of emerging two-dimensional materials.”

In a paper published in the journal Proceedings of the National Academy of Sciences (“Gate-tunable carbon nanotube–MoS2 heterojunction p-n diode”), the Northwestern team used single-walled carbon nanotubes as the p-type semiconductor and the MoS2 as n-type semiconductor.

The researchers discovered that when they stacked the two semiconductors vertically on top of each other they formed a heterojunction that allowed for the tuning of the device’s electrical characteristics with an applied gate bias.

In addition to its tunability, the p-n heterojunction diode is highly light sensitive. The researchers exploited this capability by making an ultrafast photodetector with the diode that displayed an electronically tunable wavelength response.

With 2-D materials already proving capable of making field-effect devices, it is hoped that this latest addition of a p-n junction diode made from one will mark an important step in the next generation of electronics.

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3D-Stacked CMOS Takes Moore’s Law to New Heights

When transistors can’t get any smaller, the only direction is up

10 min read
An image of stacked squares with yellow flat bars through them.
Emily Cooper

Perhaps the most far-reaching technological achievement over the last 50 years has been the steady march toward ever smaller transistors, fitting them more tightly together, and reducing their power consumption. And yet, ever since the two of us started our careers at Intel more than 20 years ago, we’ve been hearing the alarms that the descent into the infinitesimal was about to end. Yet year after year, brilliant new innovations continue to propel the semiconductor industry further.

Along this journey, we engineers had to change the transistor’s architecture as we continued to scale down area and power consumption while boosting performance. The “planar” transistor designs that took us through the last half of the 20th century gave way to 3D fin-shaped devices by the first half of the 2010s. Now, these too have an end date in sight, with a new gate-all-around (GAA) structure rolling into production soon. But we have to look even further ahead because our ability to scale down even this new transistor architecture, which we call RibbonFET, has its limits.

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