PORTLAND, Ore. Processing semiconductors at room temperature could enable large-scale applications like electronic billboards and ultra-low-cost applications like disposable RFID tags. But most room-temperature transistors have dismal electron mobility measured in the hundredths of centimeters2 per volt second (cm2/Vs).
Now, Georgia Institute of Technology researchers are claiming to have perfected a method of making room- temperature transistors that are 100-times faster--as fast a amorphous silicon--by fabricating their channels from thin films of carbon-60 (C60), also knows as buckyballs or fullerenes.
"We do not claim to be the first lab to fabricate C60 transistors at room temperature," said Georgia Tech professor Bernard Kippelen. "The novelty of our work is that we were able to demonstrate simultaneously very high mobility values of three-to-five cm2/Vs with room temperature processing, good reproducibility, good stability, low threshold voltage, and large on-off current ratios."
Laboratories worldwide are pursuing room-temperature processes so that large displays and low-cost applications like RFID tags can be made using inexpensive, high-volume roll-to-roll presses and ink-jet printing. That would eliminate the need for expensive, high-temperature processing in clean rooms. Many approaches are using organic materials for transistors while trying to find ways to increase electron mobility with new material formulations.
Others have achieved mobilities as high or higher than the Georgia Tech group, but only by resorting to high temperatures to create organic transistors. Kovio, Inc. (Sunnyvale, Calif.) has developed an inorganic silicon ink for making thin-film transistors that can be fabricated with ink-jet printing using roll-to-roll manufacturing, albeit at temperatures too high for plastic substrates.
While not achieving the electron mobilities of high- temperature approaches (Kovio claims mobilities as good as polysilicon) the Georgia Tech transistors have achieved mobilities better than amorphous silicon. One potential application is service displays that require only a 16 millisecond refresh rate. And, they can be cast on inexpensive plastic substrates.
Building on several years of design refinements, Kippelen claimed his group has identified the materials and parameters that needed to be optimized for high-electron mobilities at low temperatures. "Our work builds on years of experience with the purification and processing of organic semiconductors," said Kippelen. "The choice of the gate dielectric and the choice of the metal for the electrodes played an important role as well."
|Professor Bernard Kippelen (center) collaborated with Georgia Tech research scientist Benoit Domercq and doctoral candidate Xiao-Hong Zhang on the transistor project.|
The demonstration devices were fabricated on a silicon substrate for convenience, but the researchers claim that all the elements of their organic C60 transistors were fabricated at room temperature. The metal electrode for the transistor was deposted using the same transparent process used for organic light-emitting diodes and plastic solar cells.
"Our electrodes were fabricated on top of the organic semiconductor using shadow masking and thermal evaporation," said Kippelen. "By keeping the distance between the metal source and substrate large enough (3 feet) the substrate is not heated significantly during the deposition."
Next, the researchers will seek to demonstrate fabrication in both n-channel and p-channel transistors so that complementary circuits similar to CMOS inverters, ring oscillators, logic gates and drivers for active-matrix displays can be made using room-temperature organic materials.
"Going from silicon to plastic substrate is also part of our future research plans," said Kippelen.
One downside to using C60 for transistor channels is their sensitivity to oxygen, meaning devices must operate in a nitrogen atmosphere. The researchers plan to address this problem by reformulating the fullerene molecules and by packaging them to exclude air.