Two Good Gate Materials That Are Great Together

Okay, I couldn’t help myself. When I saw this story in which researchers at Georgia Tech had developed a top-gate organic field-effect transistor for plastic electronics that used a bilayer gate dielectric, I thought of those old Reese’s Peanut Butter Cup commercials: “Two great tastes that taste great together.”

“Rather than using a single dielectric material, as many have done in the past, we developed a bilayer gate dielectric,” said Bernard Kippelen, director of the Center for Organic Photonics and Electronics and professor in Georgia Tech’s School of Electrical and Computer Engineering.

The bilayer gate consists of a fluorinated polymer known as CYTOP and a high-k metal-oxide layer created by atomic layer deposition.

As noted in the Georgia Tech press release: “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 doesn’t have good stability because of a high number of defects on the interface.”

So, in a sort of ‘let’s give it a shot’ spirit, the researchers wondered if they combined the two materials whether they would cancel out each other’s drawbacks. Answer: Yes.

“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,” said Kippelen.

“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,” explains Kippelen.  “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 threshold voltage and over time to an increase of the current. But if you combine them, their effects cancel out.”

The results have even surpassed the researchers’ expectations. “I had always questioned the concept of having air-stable field-effect transistors, because I thought you would always have to combine the transistors with some barrier coating to protect them from oxygen and moisture. We’ve proven ourselves wrong through this work,” said Kippelen.

While the applications for the transistor run the gamut of plastic electronics, including smart bandages, RFID tags, plastic solar cells, light emitters for smart cards, the transistors have only been demonstrated to date on glass substrates.

After seeing if they can get the transistors to work on plastic substrates, they will look into whether they can manufacture them with ink jet printing.