Revolutions in OECTs
Organic electrochemical transistors (OECTs) have shown promise for different healthcare applications, from bioelectronics to biosensors and wearables. However current OECTs are subject to certain limitations, notably slow switching speeds and low temporal and operational stability. Studies are showing that a different architecture may be able to overcome some of these limitations. And this could be great news for medical applications.
OECTs Today
One of the main advantages of organic electrochemical transistors is very high transconductance. In OECTs, ions are injected into the channel and change the electronic charge density throughout. This results in higher transconductance than is seen in electrolyte-gated field effect transistors, where ions do not penetrate into the channel.
In addition to high transconductance, OECTs are intrinsically flexible, have a low working voltage, and they don’t require a lot of power. In addition, fabrication and miniaturization are fairly straightforward, OECTs are compatible with a wide range of mechanical supports such as plastic, fibers, paper, and elastomer, and they’re stable in aqueous environments.
Wearable and implantable bioelectronics are a natural fit for organic electrochemical transistors. Biosensing is another place where this technology shows great potential. Some applications for which OECTs are particularly promising include e-skin devices, electroencephalography, electrocardiography, electrooculography, and electromyography.
One downside of this technology, however, is speed. OECTs are limited by the speed of the migration of ions in and out of the channel, which makes them comparatively slow. In addition, OECTs generally exhibit low temporal and operational stability.
Planar vs. Vertical
Conventional OECTs have a planar architecture, which is similar to that of conventional transistors that incorporate inorganic semiconductors. Typically, two gold electrodes–the source and the drain–on a substrate are separated by the channel, which is made from an organic semiconductor material. The channel is covered by an electrolyte medium. A non-conducting encapsulation layer holds the electrolyte and defines the area of the channel exposed to it.
In a 2023 study published by Huang et al, researchers turned the OECT architecture on its ear, so to speak. The researchers sandwiched the channel between the source and drain. This, in turn, shortened the distance between the gold electrodes, making it equal to the thickness of the channel. They replaced the channel material with a new semiconductor polymer.
This vertical architecture, combined with the new channel material, greatly improved both the stability and the performance of the OECT. In fact, these changes resulted in the highest reported transconductance per unit area of the channel material in an OECT device, as well as the highest current in the “on” state. On top of that, the vertical n-type OECTs in the study outperformed any earlier n- and p-type OECTs when used in complementary logic circuits.
The study concluded that vertical, rather than planar structure, combined with a different channel material could potentially overcome two of the most significant problems of organic electrochemical transistors.
Challenges
Vertical architecture does, of course, present some challenges of its own.
In planar OECTs used in biosensing, for example, biomolecules are commonly anchored to either the gate or the channel. In vertical OECTs, however, the gate and channels are buried. As a result, new circuit designs will be needed to use vertical OECTs in biosensors.
Additional investigation will also be needed to determine whether vertical architecture will work with a wide range of semiconductors, to incorporate the architecture into large-area circuits, and to simplify fabrication.