Completed monolayer organic transistors with transferred electrodes. (Image courtesy of the researchers)

Field Effect Transistors (FET) are the core building blocks of modern electronics such as integrated circuits, computer CPUs, and display backplanes. Organic Field Effect Transistors (OFETs) have the advantage of being flexible when compared with their inorganic counterparts like silicon.

OFETs, given their high sensitivity, mechanical flexibility, biocompatibility, property tunability, and low cost of fabrication, have great potential in new applications such as wearable electronics, conformal health-monitoring sensors, and bendable displays. Imagine TV screens that can be rolled up; or smart wearable electronic devices and clothing worn close to the body to collect vital body signals for instant biofeedback; or mini-robots made of harmless organic materials working inside the body for disease diagnosis, targeted drug transportation, mini-surgeries and other medical applications.

Until now, the main limitation on enhanced performance and mass production of OFETs has been in the difficulty of miniaturizing them. Products using OFETs currently on the market are limited in terms of product flexibility and durability.

An engineering team led by Dr. Paddy Chan Kwok Leung at the Department of Mechanical Engineering of the University of Hong Kong (HKU) has made an important breakthrough in developing a staggered structure monolayer Organic Field Effect Transistor, which will greatly facilitate reducing the size of OFETs.

The major problem that has been confronting scientists in reducing the size of OFETs is that the performance of the transistor drops significantly with a reduction in size. This is partly due to the problem of contact resistance — resistance at the interfaces — which resists current flows. When the device gets smaller, its contact resistance becomes a dominating factor in significantly downgrading the device’s performance.

The staggered structure monolayer OFETs demonstrate a record low normalized contact resistance of 40 Ω-cm compared with conventional devices that have a contact resistance of 1000 Ω-cm. The new device can save 96% of power dissipation at the interface, thus reducing the heat generated in the system, a common problem that causes semiconductors to fail. That, in turn, will enable the dimensions of OFETs to be reduced to the sub-micrometer scale, a level compatible with their inorganic counterparts, while still functioning effectively to exhibit their unique organic properties. “That is critical for meeting the requirements for commercialization,” Dr Chan said.

These OFETs also have an improved signal to noise ratio, which will enable them to detect weak signals that could not previously be detected using conventional bare electrodes for sensing.

Flexible OFETs could transform traditional rigid devices such as display panels, computers, and cell phones, enabling them to become flexible and foldable. These future devices would also be much lighter in weight and have low cost of production.

“Moreover, given their organic nature, they are likely to be biocompatible for advanced medical applications such as sensors for tracking brain activity or neural spike sensing, and in precision diagnosis of brain related illnesses such as epilepsy.” Dr Chan added.

Dr Chan’s team is currently working with researchers at the HKU Faculty of Medicine and biomedical engineering experts at CityU to integrate the miniaturized OFETs into a flexible circuit on a polymer microprobe for neural spike detection in-vivo on a mouse brain under different external stimulations. They also plan to integrate the OFETs into surgical tools such as catheters, which can be inserted into animals’ brains for directly sensing brain activity in order to locate abnormalities.