Bob Michaels

January 31, 2011

3 Min Read
Georgia Tech Researchers Develop a Transistor Suitable for Plastic Electronics

A top-gate organic field-effect transistor with a bilayer gate insulator can be fabricated on a plastic substrate, making it suitable for such applications as smart bandages.

In the quest to develop flexible plastic electronics, one of the stumbling blocks has been creating transistors with enough stability for them to function in a variety of environments while still maintaining the current needed to power devices. Now, researchers at the Georgia Institute of Technology (Georgia Tech; Atlanta) have developed a method of combining top-gate organic field-effect transistors with a bilayer gate insulator. This technique enables stable transistor performance while exhibiting good current performance. Among its potential applications, the transistor could be suitable for manufacturing smart bandages.

As detailed in Advanced Materials, the team's transistor is based on an existing semiconductor in which they have changed the gate dielectric. "Rather than using a single dielectric material, as many have done in the past, we developed a bilayer gate dielectric," remarks Bernard Kippelen, director of the Center for Organic Photonics and Electronics and professor at Georgia Tech's school of electrical and computer engineering.

The bilayer dielectric is made of a fluorinated polymer known as CYTOP and a high-k metal-oxide layer created using atomic layer deposition, materials that have both benefits and drawbacks when used alone. CYTOP is known to form few defects at the interface of the organic semiconductor, but it also has a very low dielectric constant, which requires an increase in drive voltage. The high-k metal-oxide uses low voltage, but it does not exhibit good stability because of a high number of defects on the interface.

Thus, Kippelen attempted to combine the two substances in a bilayer, hoping that the drawbacks would cancel each other out. "When we started to do the test experiments, the results were stunning. We were expecting good stability, but not to the point of having no degradation in mobility for more than a year," Kippelen says.

To determine how stable the bilayer was, the scientists cycled the transistor 20,000 times, tested it under a continuous bias stress by running the highest possible current through it, and placed it in a plasma chamber for five minutes. In all cases, there was no degradation. They observed degradation only when the transistor was placed in acetone for an hour. Even then, it was still operational.

"By having the bilayer gate insulator, we have two different degradation mechanisms that happen at the same time, but the effects are such that they compensate for one another," Kippelen explains. "So, if you use one, it leads to a decrease of the current; if you use the other, it leads to a shift of the thereshold voltage and over time to an increase of the current. But if you combine them, their effects cancel out."

The transistor conducts current and runs at a voltage comparable to that of transistors based on amorphous silicon, the current industry standard. But it can be manufactured at temperatures below 150°C, enabling it to accommodate plastic substrates. It can also be created in a regular atmosphere, making it easier to fabricate than other transistors.

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