Organic transistors hold promise for wearable electronics, biosensors to measure sodium or other ions in blood, and other applications. But their lifetimes and switching speeds are limited. Now, researchers have boosted the performance of a family of organic transistors by flipping the conventional horizontal design on its side (Nature, 2023. DOI: 10.1038/s41586-022-05592-2).
Transistors act as switches, turning current on and off to code for the 1s and 0s of digital logic. The new work focuses on a type of device called an organic electrochemical transistor (OECT). OECTs switch by controlling the number of electrons or holes (positive charges) in a semiconducting polymer with the help of ions from an electrolyte. A voltage applied to a metallic gate electrode, which is immersed in the electrolyte, injects ions from the electrolyte into the semiconductor, changing its conductivity.
The OECT design is particularly useful for biosensors because the liquid electrolyte helps the device interface the ion world of biology with the electronic world of transistors, says Antonio Facchetti, a chemist at Northwestern University.
OECTs have low power requirements compared to other electronic switches. But they have typically had slow switching speeds, which means they can’t handle complex computing tasks, and they wear out relatively quickly. Making modern logic circuits with OECTs so far has also been difficult. Integrated circuits require two types of transistors—n-type transistors that conduct electrons and p-type devices that conduct positive charges or “holes”—but the n-type OECTs made so far have performance far lower than those of p-type counterparts, says Tobin J. Marks of Northwestern University.
Marks, Facchetti, Wei Huang of University of Electronic Science and Technology of China and their colleagues solved these issues by making new semiconducting polymers, and by changing the architecture of the device. The new p-type and n-type polymers, both based on a compound containing pentaoxahexadecan and dihydropyrrole, are better conductors than the polymers used to make OECTs so far.
Conventional OECTs are planar with charges flowing horizontally from one electrode to another through a semiconductor channel. The photolithography techniques used to make them dictate the distance between the electrodes, typically around 1–5 µm, which in turn determines the time it takes electrons to travel from one end to the other, setting a limit on switching speed.
The researchers changed to a vertical channel design, sandwiching the semiconductor between two electrodes on top and on bottom. This shrinks the distance that the charge carriers have to travel to the thickness of the semiconductor material, just 100 nm. That’s tens of times smaller than the channels in conventional planar OECTs. The resulting transistors switch on and off in less than 1 millisecond, and last for over 50,000 cycles, much higher than what has been possible before.
That performance holds for both p-type transistors and n-type transistors. Aristede Gumyusenge, a chemist at MIT says the n-type transistors in this work demonstrate the highest performance yet, opening up the possibility of using OECTs in advanced circuits for the first time. The researchers will now have to show that the vertical architecture works for a range of semiconductors, he says, and to incorporate the architecture into large-area circuits.