For decades, field effect transistors, made possible by silicon-based semiconductors, have fueled the electronics revolution. But in recent years, manufacturers have come up against hard physical limits for further silicon chip size reductions and efficiency gains. That’s why scientists and engineers are looking for alternatives to conventional metal-oxide-semiconductor (CMOS) transistors.
“Organic semiconductors offer several distinct advantages over traditional silicon-based semiconductor devices: They are made from abundant elements such as carbon, hydrogen and nitrogen; they offer mechanical flexibility and low manufacturing costs; and they can be easily manufactured on a large scale. ” notes UC Santa Barbara engineering professor Yon Visell, who is part of a group of researchers working with the new materials. “Perhaps more importantly, the polymers themselves can be manufactured using a variety of chemical methods to endow the resulting semiconductor devices with interesting optical and electrical properties. These properties can be designed, tuned, or selected in many more ways than with inorganic (e.g., silicon-based) transistors.”
The design flexibility described by Visell is exemplified by the reconfigurability of the devices, which UCSB researchers and others report in the journal Advanced Materials.
Reconfigurable logic circuits are of particular interest as candidates for post-CMOS electronics, as they allow for simplified circuit design while increasing power efficiency. A recently developed class of carbon-based (as opposed to, say, silicon- or gallium-nitride-based) transistors, referred to as organic electrochemical transistors (OECTs), has proven well suited for reconfigurable electronics.
In the latest newspaper, Chemistry professor Thuc-Quyen Nguyen, who directs the UCSB Center for Polymers and Organic Solids, and co-authors including Visell describe a breakthrough material – a soft, carbon-based semiconducting polymer – that may offer unique advantages over the inorganic semiconductors currently found in conventional silicon transistors.
“Reconfigurable organic logic building blocks are promising candidates for the next generations of efficient computing systems and adaptive electronics,” the researchers write. “Ideally, such devices would be simple in design and construction, [as well as] energy efficient and compatible with high-throughput microfabrication techniques.”
Conjugation for conductivity
A conjugated polyelectrolyte or CPE-K consists of a central conjugated backbone with alternating single and double bonds and multiple charged side chains with ions attached to them. “Conjugated bonds throughout the polymer make it conductive because the delocalized electrons have high mobility along the length of the polymer,” explains lead author Tung Nguyen-Dang, a postdoctoral fellow in Nguyen’s lab who is advised by Visell. “They marry two classic materials, the polymer and the semiconductor, in this molecular design.”
Artificial intelligence (AI) played a role in the development of the material. “You can use trial and error to create a material,” says Nguyen. “You can make a whole bunch of these and hope for the best, and maybe one in twenty will work or have interesting properties; however, we worked with Gang Lu, a professor at California State Northridge, who used AI to select building blocks and do calculations to get a rough idea of how we would, given the energy levels and properties we strive, shall proceed.”
Find out reconfigurability
A key benefit of CPE-K is that it allows for reconfigurable (“dual-mode”) logic gates, meaning they can operate in either depletion mode or accumulation mode on the fly, simply by adjusting the voltage on the gate. In depletion mode, the current flowing through the active material between the drain and source is initially high before a gate voltage is applied (also known as the ON state). When the gate voltage is applied, the current drops and the transistor is switched to an OFF state. The accumulation mode is the opposite – with no gate voltage, the transistor is in an OFF position, and applying a gate voltage results in a higher current, switching the device to an ON state.
“Traditional electronic logic gates, which are the building blocks for all digital circuitry in computers or smartphones, are hardware that does just the one job it was designed to do,” says Nguyen. “For example, an AND gate has two inputs and one output, and if the inputs applied to it are all 1, then the output is 1. Likewise, a NOR gate also has two inputs and one output, but if all the inputs applied to it are 1, then the output is 0. Electronic gates are implemented with transistors, and their reconfiguration (e.g. changing from an AND gate to a NOR gate) requires invasive modifications, such as B. a disassembly, which is usually too complicated to be practical.
“Reconfigurable gates, like the one shown here, can behave like both types of logic gates and switch from AND to NOR and vice versa just by changing the gate voltage,” she continues. “Currently, functionality in electronics is defined by structure, but in our device you can change the behavior and make something else out of it simply by changing the voltage applied to it. If we scale up this invention from a single gate to much more complex circuits among many such reconfigurable gates, we can envision a powerful piece of hardware that can be programmed with many more functionalities than traditional gates with the same number of transistors.”
Another advantage of CPE-K-based OECTs: they can be operated with very low voltages, which makes them suitable for use in personal electronics. This, combined with its flexibility and biocompatibility, makes the material a likely candidate for implanted biosensors, wearable devices, and neuromorphic computing systems in which OECTs could serve as artificial synapses or non-volatile memories.
“Our colleague makes devices that can monitor the drop in glucose levels in the brain that occurs just before a seizure,” explains Nguyen of a collaborator 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.”
According to Nguyen, devices made of CPE-K have simultaneous doping and undoping depending on the type of ions. “You make the device and put it in a liquid electrolyte — sodium chloride [i.e., table salt] dissolved in water,” she says. “You can then get the sodium to migrate into the CPE-K active layer by applying a positive voltage to the gate. Alternatively you can change the polarity of the gate voltage and cause chloride to migrate to the active layer. Each scenario creates a different type of ion injection, and these different ions allow us to change the operating modes of the device.”
Auto-doping also simplifies the fabrication process by eliminating the extra step of adding dopants. “Often when you add a dopant, it’s not evenly distributed throughout the bulk of the material,” says Nguyen. “The organic dopants tend to clump together rather than disperse. However, since our material does not require this step, the problem of non-uniform doping distribution does not arise. You also avoid the whole process of doping optimization and determining proper mix and proportions, all of which add steps and complicate processing.”
The team also developed a physics model for the device that explains its working mechanism and correctly predicts its behavior in both modes of operation, showing that the device is doing what it appears to be doing.
Visell concludes: “This remarkable new transistor technology is an ideal example of the surprising electronic and computational functionalities made possible by convergent research in chemistry, physics, materials and electrical engineering.”