A heat-gated transistor in which an electric current can be modulated by changing the temperature of the gate electrode has been developed by researchers in Sweden. The device combines two developing scientific fields – ionic thermoelectrics and polymer electronics – and could have a wide range of potential applications, from medical imaging to night vision.

Temperature is a key physical quantity that is measured in many fields of science and technology. Infrared binoculars and cameras measure temperature differences and are used for night vision, whereas mapping the temperature of tissue can provide important medical information. Despite its importance, mapping temperature changes in space and time can be very challenging.

One option is to use thermoelectric sensors called thermocouples, in which two different materials develop a potential difference in response to a temperature difference. The voltage produced in traditional thermoelectric materials is usually small, however, which limits the sensitivity of the detectors. Multiple thermocouples connected in series are required to provide the gate voltage for a transistor, which would then convert a small heat signal to a signal that could be displayed in a device. This makes the circuitry in imaging devices complex and bulky.

Researchers at Linköping University in Sweden have overcome this problem using an ionic thermoelectric polymer electrolyte. Traditional thermoelectric materials – which conduct electricity through the temperature-induced motion of either electrons or holes – usually achieve a maximum potential difference of a few hundred microvolts per degree kelvin. In contrast, the Linköping group is developing electrolytes in which charge separation is achieved by the motion of ions.

Mobile ions

In 2016, the team developed an electrolyte containing a solution of the polymer polyethyleneoxide. When sodium hydroxide is added to the solution, the hydroxide ions combine with protons from alcohol groups on the polymer chains to create a solution of mobile sodium ions and relatively stationary, negatively charged polymer chains. When one end of the electrolyte heats up, the positive sodium ions diffuse away from the heat faster than the polymer chains, creating a negative charge at the hot end. The thermoelectric effect in these ionic electrolytes can be much stronger than in conventional materials – as much as 11,000 μV K–1. The researchers injected this solution between two electrodes to produce a thermoelectric "supercapacitor" that could charge up during the day and produce electricity at night.

In the new research, the team has integrated the supercapacitor into a polymer transistor so that one of the supercapacitor electrodes functions as the transistor's gate electrode. Team member Simone Fabiano explains that the second key innovation lies here: "The transistor we are using is an electrolyte-gated transistor," he explains, "And the beauty is that you can have a modulation of the current on a gate-voltage range which is much smaller than typical transistors."

Applying heat to the back electrode of the supercapacitor changes the voltage on the gate electrode and alters the resistance between the source and drain electrodes of the transistor. By combining a thermoelectric sensor that can produce much larger voltages than usual with a transistor that can operate at much smaller voltages, the researchers removed the need for multiple thermocouples. Instead, a detectable change in the current is produced simply by changing the temperature of one electrode by one degree. This could make it much easier to produce arrays of detectors for imaging, for example.

Robot skin

As a bonus, polymer transistors can be made flexible and stretchable and can easily be printed on skin and a variety of other substrates. This could prove useful for making "electronic skin" – networks of tiny sensors that can wrap around objects such as human skin and map temperature variations. "You could get clinically relevant medical information," explains Fabiano. "You could track a healing process or get information about pathological conditions that are directly related to variations in body temperature." Electronic skin could also be useful in robotics.

"It's enabling for a lot of applications," says engineer George Malliaras of MINES Saint-Etienne in France. He adds: “The researchers produced something that can easily be microfabricated and placed on large areas. It's early stage work and the limits need to be explored, but I see this as a very promising technology that can take many forms. I look forward to seeing what they will cook up next!"

The research is published in Nature Communications.