The transistor is one of the most important components in electronics. By switching or amplifying electrical signals, transistors make sure that our smartphones, computers and even coffee machines do what we want them to do. A new generation of energy-efficient transistors is now being developed based on thin, lightweight, electrically conducting films. Imagine miniature actuators with their own built-in sensing capability integrated into flexible, film-based circuit boards. This is the vision being pursued by research teams led by Paul Motzki (Saarland University) and John Heppe (University of Applied Sciences, htw saar). The team will be showcasing the principle behind this innovative film-based switch at the Hannover Messe technology fair from 20 to 24 April (Hall 11, Stand D41).
A common way of explaining what transistors do is to say that transistors are to electricity what a tap is to water. Picture electricity as water rushing through a pipe, where the pressure driving the flow is the applied electrical voltage. A transistor can switch the flow on and off or regulate how much current is allowed through. Without transistors, modern life would grind to a halt. The numbers involved are staggering: a single smartphone contains many billions of transistors. But when high voltages and high frequencies are involved, transistors can become large and expensive. Today’s transistors are built from multiple layers of semiconductor materials such as silicon or germanium.
Researchers in Saarbrücken are now breaking new ground by building transistors from silicone film. ‘We replace the semiconductor components with materials known as dielectric elastomers,’ explains Paul Motzki, Professor of Smart Material Systems at Saarland University, who is developing the concept together with Professor John Heppe from htw saar. The two teams will be exhibiting a demonstration version of their novel film-based electrical switch at this year’s Hannover Messe.
Smart films instead of silicon
The two research groups specialize in turning ultrathin polymer films into actuators that operate without the need for additional sensors. The films are coated on both sides with a highly flexible, electrically conductive electrode layer. When the researchers apply a voltage to the elastomeric polymer film, these electrically conducting layers attract each other, compressing the film and causing it to expand out sideways, thereby increasing its surface area. ‘By varying the applied voltage, we can control the motion of the film very precisely and make it execute continuously variable flexing motions or make it vibrate at some desired frequency or amplitude,’ explains Paul Motzki. The films are also self-sensing, because even the smallest movement of the film corresponds to a specific change in electrical capacitance. Using these capacitance values, the researchers are able to precisely quantify the spatial deformation of the film. This property also allows them to control the motion of the film, specifying precisely how it should deform, and how quickly.
For years, Paul Motzki and his team have been refining these smart films so that they respond ever faster and ever more sensitively. The result is a new class of energy-efficient miniature actuators that can be controlled so as to perform complex, highly precise motion sequences or to hold a defined fixed position. The team has already built a wide range of prototypes – from a tactile ‘second skin’ that can be integrated into wearable garments, virtual dynamic buttons that provide haptic feedback to touchscreen users, to pumps and valves, and even ultra-lightweight loudspeakers. And these film-based actuators only consume energy while they are moving, not while they are holding a fixed position.
Elastic films that can control the flow of electric current – actuator and sensor in one
The next step is to make these smart films function as electronic switches, so that in future they can be used as energy-efficient transistors. To do this, the films need a new type of coating. Until now, the conductive layer has been made from powdered amorphous carbon, known as ‘carbon black’. However, the electrical resistance of the carbon black layer is far too high for efficient transistor applications. That is why Paul Motzki’s team is working with Professor John Heppe’s research group ‘Physical Sensors and Mechatronics’ at htw saar. Both groups are working side-by-side at the Center for Mechatronics and Automation Technology in Saarbrücken (ZeMA) – a joint research facility operated by the two universities, with Paul Motzki as ZeMA’s Scientific Director/CEO.
John Heppe’s team specializes in ultrathin coatings. Instead of applying a coat of carbon black, the team is researching ways to apply a highly conductive metal layer that will switch current on and off at extremely high speeds. The challenge is that the film must still be able to stretch significantly in order to work, which means that two rigid, printed metal layers are not really going to work. The solution, it turns out, is to use a coating that is not rigidly printed onto the film. ‘We use a specialized coating technique known as sputtering,’ says John Heppe. The trick is that the researchers first stretch the film before depositing an ultrathin conductive metal layer. With a thickness of around ten nanometres, this electrode layer is more than a thousand times thinner than a human hair. They then stretch the film a little further. ‘It’s like painting a circle on a balloon: if you continue to inflate it, the painted layer will start to tear in many places,’ explains Heppe.
An ultrathin coating of metal full of cracks – the route to a flexible film-based transistor
When the researchers release the tension, the cracks close up and current is able to flow. When they stretch the film, the cracks in the electrode layer open up again and the current is interrupted, which offers controlled on-and-off switching – exactly as required. If the metal layer on the elastomer contracts in such a way that it forms folds, it still offers only minimal resistance, allowing large currents to flow through the film switch. ‘We see resistances of just 50 to 100 ohms over an area of roughly one square centimetre, which means that the high voltage circuits with very fast switching cycles needed to control valves and pumps are also a realistic option. We can switch from very low to very high resistances,’ says Professor Heppe. Between the two extremes (‘all cracks open’ and ‘all cracks closed’), the amount of current that flows can be precisely controlled – just like a tap regulates the flow of water. Electrodes of this kind can be sputtered onto the polymer film at spacings of just a few micrometres.
‘In future, film transistors could be used for higher frequency switching of voltages in the kilovolt range – something that currently requires some large and often costly transistor components. Overall, these switching systems would become smaller, more efficient and cheaper,’ says Paul Motzki. The Saarbrücken research teams are aiming to develop lightweight, flexible film-based circuit boards for high-voltage applications. Until now, the signals that control electrical and electronic devices have been regulated by conventional transistors soldered onto flat, rigid circuit boards. With this new technology, transistors could be integrated directly into flexible actuator films. ‘Our innovative flexible circuit boards could open up new applications in fields such as medical technology. Miniature self-sensing actuators would be incorporated directly into the film circuit board,’ says Motzki.
At Hannover Messe, the teams will showcase their technology with a demonstration film switch : When a lever is pulled, the polymer film stretches, cracks form in the ultrathin metal electrode layer, the resistance jumps abruptly from the ohm range into the high megaohm range, and the flow of current ceases. When the lever is released, the film relaxes, the cracks close up and – with the resistance now back down at below 100 ohms – current flows with minimal losses, even at voltages of up to, say,10 kilovolts, which is the sort of voltage level needed for novel electrostatic actuators.
Background
The TransDES project (Researching novel Trans istor structures based on flexible D ielectric E lastomer S ystems) is funded jointly by Saarland and the European Regional Development Fund (ERDF). The project is a collaboration between research teams led by Professor Paul Motzki (Saarland University) and Professor John Heppe (htw saar) at the Center for Mechatronics and Automation Technology (ZeMA) in Saarbrücken – a joint research facility run by both universities.
Dielectric elastomer technology continues to be explored and developed in numerous doctoral research projects. The work has been published in a variety of specialist journals and has received support from numerous sources, including EU funding via a Marie Curie Research Fellowship and the German Research Foundation through its DFG Priority Programme SPP KOMMMA. The Saarland state government has provided financial support through the ERDF projects iSMAT and Multi-Immerse, and ME Saar (the Association of Metalworking and Electrical Industries in Saarland) has funded a doctoral research programme.
The researchers are translating the results of their application-driven research into industrial practice. To facilitate the transfer of their smart materials technology into the commercial and industrial sectors, the researchers established the company ‘mateligent GmbH’, which will also be exhibiting at the same stand at this year’s Hannover Messe.
https://imsl.de – Intelligent Material Systems Lab
https://zema.de – Center for Mechatronics and Automation Technology (ZeMA)