Researchers have been seeking new organic materials for electronic components to drive down the costs and the environmental impact of manufacturing current devices as well as to meet particular advanced performance needs. Conventional metal-oxide semiconductor (CMOS) transistors are one of the devices that has reached a material impasse of sorts, with silicon—the current material of choice—facing limits in terms of the current need for simultaneous reduction in size and increased performance.
Researchers at the University of California, Santa Barbara (UCSB), have come up with an alternative to silicon in these devices with a new, soft, semiconducting carbon-based polymer that they said is well suited for alternatives to CMOS devices called organic electrochemical transistors (OECTs). They developed an OECT using the material to demonstrate its advanced functionality. OECTs devices are well-suited for use in reconfigurable electronics, which could spur the next generation of efficient computing systems and adaptive electronics, researchers said.
“Organic semiconductors offer several distinct advantages over conventional silicon-based semiconducting devices,” explained UCSB engineering professor Yon Visell in UCSB’s The Current. “They are made from abundantly available elements, such as carbon, hydrogen and nitrogen; they offer mechanical flexibility and low cost of manufacture; and they can be fabricated easily at scale.”
Moreover, the polymers themselves can be crafted using a wide variety of chemistry methods to create semiconducting devices with “interesting optical and electrical properties,” he noted in the article. “These properties can be designed, tuned or selected in many more ways than can inorganic (e.g., silicon-based) transistors.”
Advanced Electronic Material
The material developed by the team is called a conjugated polyelectrolyte, or CPE-K, which is comprised of a central conjugated backbone, with alternating single and double bonds, and multiple charged side chains with ions attached.
“Having conjugated bonds throughout the polymer makes it conductive, because the delocalized electrons have high mobility across the length of the polymer,” explained Tung Nguyen-Dang, a postdoctoral researcher in the lab of project leader Thuc-Quyen Nguyen, who also leads the UCSB Center for Polymers and Organic Solids. “You are marrying two classic materials, the polymer and the semiconductor, in this molecular design,” she stated in the article.
CPE-K has a number of benefits for the components made with organic transistors, particularly for reconfigurable logic devices, or logic gates, researchers said. One benefit is that it enables reconfigurable, or “dual-mode,” logic gates, meaning they can be switched on the fly to operate in either depletion mode or accumulation mode, simply by adjusting the voltage at the gate, researchers said.
In depletion mode, current flowing through the active material between the drain and the source is initially high, before application of any gate voltage—or the “On” state, researchers said. When the gate voltage is applied, the current drops and the transistor is turned to an “Off” state.
Accumulation mode is the opposite—without gate voltage, the transistor is in an Off position, and applying a gate voltage yields higher current, switching the device to an On state.
Reconfigurable logic gates, in contrast to currently manufactured logic devices, can behave as both types of logic gates, switching back and forth between states by changing only the gate voltage, researchers said.
In theory, these components can be scaled up from a single gate to much more complex circuits consisting of many such reconfigurable gates, they said. This paves the way for far more powerful devices than the ones that currently exist—which are generally designed to perform one core function. Reconfigurable logic gates, in contrast, can be programmed with multiple functionalities using the same number of transistors, researchers said.
Another benefit to CPE-K in its use in OECTs is that it allows the devices to be operated at very low voltages, making them suitable for use in personal electronics, researchers said. The material is also flexible and biocompatible, suiting it for implanted biosensors, wearable devices, and neuromorphic computing systems in which OECTs might serve as artificial synapses or non-volatile memories, they said.
In fact, other researchers working with the UCSB team already are developing devices that can benefit from transistors made using the carbon-based polymer, Nguyen said. “Our colleague is making devices that can monitor the drop of glucose level in the brain that occurs just before a seizure,” she said of a scientist at the University of Cambridge in England. “And after detection, another device—a microfluidic device—will deliver a drug locally to stop the process before it happens.”
Blueprint for Next-Gen Electronics
Researchers published a paper on their work in the journal, Advanced Materials.
In addition to developing a novel transistor, researchers also developed a physics model for the device that explains its working mechanism and correctly predicts its behavior in both operation modes, thus demonstrating that the device is doing what it seems to be doing.
Researchers said their work demonstrates what’s possible in the future when research in chemistry, physics, materials, and electrical engineering converges to develop next-generation electronics and computing devices.