Semiconductor-free logic gates pave the way for fully 3D-printed active electronics

Active electronics (the components that can control electrical signals) typically contain solid-state devices that receive, store, and process information. These components, which must be manufactured in a clean room, require advanced manufacturing technology that is not widely available outside of a few specialized manufacturing centers.

During the COVID-19 pandemic, the lack of large-scale semiconductor manufacturing facilities was one of the causes of a global shortage of electronic products, driving up costs for consumers and had implications for everything from economic growth to national defense. The ability to 3D print an entire active electronic device without the need for semiconductors could bring electronic manufacturing to businesses, labs and homes around the world.

Although this idea is still a long way off, MIT researchers have taken an important step in this direction by demonstrating fully 3D printed resettable fuses, which are key components of active electronics that typically require semiconductors. The study is published in the journal Virtual and Physical Prototyping.

The researchers’ semiconductor-free devices, which they produced using standard 3D printing equipment and an inexpensive, biodegradable material, can perform the same switching functions as semiconductor transistors. conductors used for processing operations in active electronics.

Although still far from reaching the performance of semiconductor transistors, 3D printed devices could be used for basic control operations like regulating the speed of an electric motor.

“This technology has real strengths. Even if we cannot compete with silicon as a semiconductor, our idea is not necessarily to replace what already exists, but to push 3D printing technology towards “In a nutshell, it’s really about democratizing technology that could allow anyone to create smart hardware far from traditional manufacturing hubs,” says Luis Fernando Velásquez-García, senior researcher at Microsystems Technology Laboratories. (MTL) from MIT and lead author of the article.

He is joined in the paper by lead author Jorge Cañada, a graduate student in electrical engineering and computer science.

An unexpected project

Semiconductors, including silicon, are materials whose electrical properties can be adapted by adding certain impurities. A silicon device can have both conductive and insulating regions, depending on how it is designed. These properties make silicon ideal for producing transistors, which are a basic building block of modern electronics.

However, researchers have not sought to 3D print devices without semiconductors that could behave like silicon-based transistors.

This project grew out of another in which they made magnetic spools through extrusion printing, a process in which the printer melts the filament and shoots the material through a nozzle, thus making an object layer by layer.

They observed an interesting phenomenon in the material they used, a polymer filament doped with copper nanoparticles.

If they passed a large amount of electrical current through the material, it would exhibit a huge spike in resistance but would return to its original level shortly after the current flow stopped.

This property allows engineers to make transistors that can function as switches, something typically only associated with silicon and other semiconductors. Transistors, which turn on and off to process binary data, are used to form logic gates that perform the calculation.

“We saw that this was something that could help take 3D printing hardware to the next level. It offers a clear way to provide a certain degree of intelligence to an electronic device,” says Velásquez-García .

The researchers attempted to reproduce the same phenomenon with other 3D printing filaments, by testing polymers doped with carbon, carbon nanotubes and graphene. Ultimately, they couldn’t find another printable material that could function as a resettable fuse.

They hypothesize that copper particles in the material spread when it is heated by electric current, causing a resistance spike that drops back down as the material cools and the copper particles get closer together. They also believe that the polymer base of the material changes from crystalline to amorphous when heated, then becomes crystalline again when cooled – a phenomenon known as the positive temperature coefficient of the polymer.

“Right now, this is our best explanation, but it’s not the complete answer because it doesn’t explain why this only happened in this combination of materials. We need to do more research, but there is no doubt that this phenomenon is real,” he says.

Active electronics by 3D printing

The team exploited this phenomenon to print switches in a single step that can be used to form logic gates without semiconductors.

The devices are made from thin 3D printed traces of the copper-doped polymer. They contain intersecting conductive regions and allow researchers to regulate resistance by controlling the voltage injected into the switch.

Although the devices don’t work as well as silicon-based transistors, they could be used for simpler control and processing functions, such as turning a motor on and off. Their experiments showed that even after 4,000 switching cycles, the devices showed no signs of deterioration.

But there are limits to the size of the changes researchers can make, depending on the physics of extrusion printing and the properties of the material. They could print devices a few hundred microns in size, but transistors in advanced electronics are only a few nanometers in diameter.

“The reality is that there are many engineering situations that don’t require the best chips. Ultimately, all you care about is whether your device can accomplish the task. This technology is capable to satisfy a constraint like that,” he said.

However, unlike semiconductor manufacturing, their technique uses biodegradable material and the process uses less energy and produces less waste. The polymer filament could also be doped with other materials, such as magnetic microparticles, which could enable additional functionality.

In the future, researchers want to use this technology to print fully functional electronic components. They are working to make a working magnetic motor using only extrusion 3D printing. They also want to refine the process so they can build more complex circuits and see how far they can push the performance of these devices.

“This paper demonstrates that active electronic devices can be fabricated using extruded polymer conductive materials. This technology allows electronics to be integrated into 3D printed structures. One interesting application is on-demand 3D printing of mechatronic components on board spacecraft,” says Roger Howe, William E. Ayer, professor emeritus of engineering at Stanford University, who was not involved in this work.

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and education.