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Boffins fabricate the 'most complex bendy microprocessor yet'

Molybdenum disulfide could be potential alternative to silicon in sensors, clothes, etc

A team of engineers from the Vienna University of Technology in Austria has created what they claim is the most complex flat and flexible microprocessor to date – using a molybdenum disulfide semiconductor.

In other words, it's a demonstration of a potentially viable alternative to silicon. The keyword here is alternative, as opposed to a replacement.

Silicon is at the heart of almost all electronic devices, and engineers are constantly shrinking circuits printed on the material to cram more transistors onto dies and more dies onto wafers. Intel reckons it can squeeze up to 100.8m 10nm FinFET transistors onto a single square millimeter. To do this, it has to build up 3D transistors. The resulting chips are powerful but not particularly bendy or flexible, instead entombed in chunky packages often with fans and heatsinks close by.

The Vienna team is keeping it strictly 2D: a simple low-density design in comparison, but one that can be printed onto plastic film and used to slip miniature electronics into clothes, tiny objects and the Internet of Things.

Their paper, published on Tuesday in Nature Communications and available for free on arXiv, shows how a molybdenum disulfide semiconductor packed with 115 transistors measuring about 280nm thick, can be integrated into a microprocessor 0.6mm2 in size. It can execute programs stored in an external memory and perform logical operations. Crucially, as we said, the transistors are 2D and flat as opposed to today's intricate 3D FinFETs designs.

This primitive layout means the MoS2 transistors can be printed on a thin-film plastic and embedded into devices a lot more easier than silicon chip packages. The paper's abstract explains:

Since then, microprocessors have been made almost exclusively from silicon, but the ever-increasing demand for higher integration density and speed, lower power consumption and better integrability with everyday goods has prompted the search for alternatives. Germanium and III–V compound semiconductors are being considered promising candidates for future high-performance processor generations and chips based on thin-film plastic technology or carbon nanotubes could allow for embedding electronic intelligence into arbitrary objects for the Internet-of-Things.

Here, we present a 1-bit implementation of a microprocessor using a two-dimensional semiconductor—molybdenum disulfide. The device can execute user-defined programs stored in an external memory, perform logical operations and communicate with its periphery. Our 1-bit design is readily scalable to multi-bit data. The device consists of 115 transistors and constitutes the most complex circuitry so far made from a two-dimensional material.

These MiS2 semiconductors won't replace silicon anytime soon, as the number of transistors pales in comparison to the billions squeezed into the latest microprocessors today. The described system is only a one-bit CPU with four possible instructions, however, we're told the blueprints can be ramped up to a multi-bit design.

Schematic of MoS2 semiconductor and its place in the design of a flat microprocessor (Image credit: Wachter et al and Nature Communications)

“We estimate that scaling our design to simple 8-bit processors should be rather straightforward, provided that the uniformity of the MoS2 films can be improved,” Thomas Mueller, coauthor of the paper and an associate professor at Vienna University of Technology, told The Register.

"Going to more complex circuits will require to replace our current NMOS-design with CMOS. Also the contact resistance of MoS2 transistors needs to be reduced to obtain good current saturation (which is required for circuits) in transistors with tens of nanometer channel lengths."

A major roadblock in the feasibility of silicon alternatives is in manufacturing. Today's silicon-churning microchip factories cost billions to set up and operate, and new materials will have to be compatible with the manufacturing systems.

We're told that the MoS2 chips can be built using standard semiconductor etching processes. Unfortunately, fabricating molybdenum disulfide components right now is trickier than using silicon: the design at this stage isn't particularly tolerant of imperfections in the material, resulting in low yields.

"As our circuits were made more or less by hand in the lab, such complex designs are of course pretty much beyond our capability. Every single one of the transistors has to function as planned in order for the processor to work as a whole," Mueller explained.

There are other advantages to having a flat semiconductor. It means the material is more flexible, making it useful for things like medical sensors or bendable displays. All in all, this is still very much an interesting and definite work in progress. ®

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