November 26, 2007 – Researchers at Georgia Tech say they’ve fabricated high-performance field-effect transistors (FET) using thin films of Carbon 60 (aka “fullerenes”) with better electron mobility than amorphous silicon, low threshold voltages, large on-off ratios, and high operational stability. The devices target potential applications for large area, low-cost electronic circuits on flexible organic substrates, including displays, active electronic billboards, and RFID tags.
“If you open a textbook and look at what a thin-film transistor should do, we are pretty close now,” said prof. Bernard Kippelen, in a statement, adding that “we have everything ready” for future work to demonstrate functional devices combining multiple components.
The transistors were made by depositing C60 molecules from the vapor phase into a thin film atop a silicon substrate, onto which a gate electrode and gate dielectric had already been fabricated. Source and drain electrodes were then deposited on top of the C60 films through a shadow mask. Mobility was shown to be lower than the previous maximum of C60 fullerenes (2.7-5cm2/V/s, vs. up to 6cm2/V/s), but the GaTech processing can be done at room temperature instead of a hot-wall epitaxy process requiring temps of 250deg. C, too hot for most flexible plastic substrates.
Due to sensitivity with oxygen these new C60 transistors must operate in a nitrogen atmosphere, but exploring other fullerene molecules should enable simpler packaging, the researchers noted. Silicon fabrication was chosen “for convenience,” they added, as a simpler step before making the devices with organic substrates.
The next stage of work the C60 FETs will produce more complex electronic components such as inverters, ring oscillators, logic gates, and drivers for active matrix displays and imaging devices. Kippelen noted that the fullerene FETs won’t compete with powerful and relatively low-cost CMOS devices, but noted that they have a niche in large-area applications where CMOS is not economical. Demonstrating “good reproducibility, good stability, near-zero threshold voltages, large on-off current ratios and performance levels higher than amorphous silicon […] may convince designers to consider this technology,” he stated.