Organic transistor stacks high

Vertical instead of planar: A new type of structure makes organic transistors many times faster and more powerful than before, as a prototype demonstrates. In it, the source and gain electrodes are on top of each other instead of next to each other. A polymer thin film forms the separating layer. This comparatively simple conversion ensures that the new vertical transistor shows the highest amplifier performance and fastest switching speed of all previously common electrochemical transistors, as researchers report in “Nature.

Without transitors , our everyday life would look very different today. Because these electrically controllable switches and amplifiers form the basis of all computers and electronic circuits. These are mostly inorganic transistors based on the semiconductor silicon. However, for some applications such as wearable electronics, sensors implanted in the body or printed circuit boards , current transistors are too inflexible and not biocompatible enough.

Organic components instead of silicon

That is why scientists have been working on organic electrochemical transistors (OECT) for a long time. These have the same construction principle as common field effect transistors, but use organic semiconductors in the form of polymers as a connecting channel between the source and drain electrodes. A liquid electrolyte connects to the gate electrode. They can be used to regulate whether and how much current flows between the other two electrodes.

“The organic electrochemical transistors have great potential because they require an exceptionally low drive voltage and low energy, but have high transconductance and biocompatibility,” explain Wei Huang from Northwestern University in Evanston and his colleagues. The transconductance is the ratio of input voltage to output current and thus the amplifier performance of the transistor.

The problem, however, is that the performance and switching speed of these organic transistors have so far been too low. It is true that their transconductance can be increased by reducing the distance between source and drain or by thickening the contact zone to the channel. So far, however, this has been at the expense of the switching speed.

On top of each other instead of next to each other

A new type of construction for the organic transistor now offers a way out of this dilemma. To do this, Huang and his team simply rotated the classic planar arrangement of source, channel and drain by 90 degrees. The two electrodes, made of thin gold foil, lie vertically on top of each other and are separated from each other by a wafer-thin layer of polymer semiconductor. This three-piece sandwich is encased in the electrolyte that connects it to the gate electrode.

The trick here is that this vertical arrangement allows the channel to be long but very narrow. This minimizes the distance between the source and drain electrodes without sacrificing switching speed. In addition, this type of construction can be manufactured using common microfabrication methods, as the team reports.

More power and more speed

Huang and his colleagues have already examined how well the new vertical transistor works in practical tests with prototypes. Accordingly, their electrochemical transistor achieves a transconductance of 0.2 to 0.2 Siemens at high switching speeds. According to the researchers, this is up to 100 times more than in conventional organic electrochemical transistors. Even with a control voltage of just 0.001 volts, it still shows good switching behavior.

“Our electrochemical vertical transistor thus achieves a whole new level of performance,” says co-author Tobin Marks from Northwestern University. “It has all the properties of a conventional organic transistor, but with a much higher transconductance, an extremely stable switching cycle, small footprint and easy, low-cost production.” manufacture transistor.

Compact circuits

And the new vertical design is also suitable for circuits, because the transistors can be combined with one another to save space. “The vertical transistors can simply be stacked on top of each other,” explains Marks’ colleague Antonio Facchetti. “This allows us to construct very compact electrochemical circuits, which is impossible with planar electrochemical ones.”

According to the research team, this new electrochemical transistor opens up new application possibilities, including in biomedical sensors that are placed on the skin or implanted in the body. (Nature, 2023; doi: 10.1038/s41586-022-05592-2 )

Source: Northwestern University