On Wednesday, a team of researchers from China used a paper published in Nature to describe a 32-bit RISC-V processor built using molybdenum disulfide instead of silicon as the semiconductor. For those not up on their chemistry, molybdenum disulfide is a bit like graphene: a single molecule of MoS₂ is a sheet that is only a bit over a single atom thick, due to the angles between its chemical bonds. But unlike graphene, molybdenum disulfide is a semiconductor.

The material has been used in a variety of demonstration electronics, including flash storage and image sensors. But we've recently figured out how to generate wafer-scale sheets of MoS₂ on a sapphire substrate, and the team took advantage of that to build the processor, which they call RV32-WUJI. It can only add single bits at a time and is limited to kilohertz clock speeds, but it is capable of executing the full RISC-V 32-bit instruction set thanks to nearly 6,000 individual transistors.

Going flat

We've identified a wide range of what are termed 2D materials. These all form repeated chemical bonds in more or less a single plane. In the case of graphene, which consists only of carbon, the bonds are all in the same plane, meaning the molecule is as thick as a carbon atom. Molybdenum disulfide is slightly different, as the angle of the chemical bonds is out of plane, resulting in a zig-zag pattern. This means the sheet is slightly thicker than its component atoms.

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Silicon chip manufacturers like Intel and TSMC are constantly outdoing themselves to make ever smaller features, but they are getting closer to the physical limits of silicon.

“We already have very, very high density in silicon-based architectures where silicon performance degrades sharply,” said Ki Seok Kim, a scientist working at the Massachusetts Institute of Technology’s Research Laboratory of Electronics.

One way around this problem is to replace silicon with graphene-like 2D materials that maintain their semiconducting properties even at a single-atom scale. Another way is building 3D chips, which squeeze more transistors into the same area without making transistors smaller. Kim’s team did both, building a 3D chip out of vertically stacked 2D semiconductors.

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