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In what could only be described as ‘fervent scientist madness’, researchers at Georgia Tech have developed a transistor with excellent stability and performance for use on plastic electronics. In addition, it can be manufactured at relatively low temperatures in a regular atmosphere. 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 the devices. The researchers describe a new method of combining top-gate organic field-effect transistors with a bilayer gate insulator. This allows the transistor to perform with incredible stability while exhibiting good current performance. The team used an existing semiconductor and changed the gate dielectric because transistor performance depends not only on the semiconductor itself, but also on the interface between the semiconductor and the gate dielectric. Instead of using a single dielectric materia, the team developed a bilayer gate dielectric. The bilayer dielectric is made of a fluorinated polymer known as CYTOP and a high-k metal-oxide layer created by atomic layer deposition. Used alone, each substance has its benefits and its drawbacks. 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 the team decided to combine the two to see what would happen. They cycled the transistors 20,000 times. There was no degradation. They tested it under a continuous biostress where they ran the highest possible current through it. There was no degradation. They even stuck it in a plasma chamber for five minutes. There was still no degradation. “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. 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,” said Bernard Kippelen, director of the Center for Organic Photonics and Electronics and professor in Georgia Tech's School of Electrical and Computer Engineering.


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