Drop a laptop computer--oops!--and the glass and brittle semiconductors of its screen may shatter. For the clumsy techie, an all-plastic display would be more durable as well as cheaper, but so far polymer transistors have required high voltages--not the kind of battery anyone would want to lug around. Now researchers report in today's Science that by simply changing an insulating material in a polymer transistor, they have cut the voltage it needs to a level comparable with the voltage in today's displays.
Polymer transistors have the same sandwich design as conventional devices, starting with a substrate carrying a metal electrode called a gate. Over the gate and substrate goes a layer of insulator followed by a layer of organic semiconductor topped off by two more contacts, one on either side of the buried gate, known as the source and the drain. Normally, a voltage between the source and drain will produce only a trickle of current. Applying a voltage to the gate, however, attracts so-called charge carriers into the region above the gate, where they allow a freer flow of current from the source to the drain. Although this gate voltage effectively turns on the transistor, plastic transistors have required voltages in the region of 100 volts to flip the switch.
A team of researchers, led by Christos Dimitrakopoulos of IBM's T. J. Watson Research Center in Yorktown Heights, New York, thought they could cut this voltage by replacing the silicon dioxide gate insulator with barium zirconate titanate, an insulator that transmits an electric field four times more efficiently. The stronger electric field would attract more charge carriers into the gate region at a given voltage, they reasoned. With the new insulator, it took a change in gate voltage of just a few volts to alter the source-drain current by more than five orders of magnitude. The performance of these transistors now rivals that of the amorphous-silicon transistors, the type of low-cost transistor used in active-matrix displays, says Dimitrakopoulos.
"This is excellent work," says plastic transistor pioneer Francis Garnier of the CNRS Laboratory of Molecular Materials in Thiais, France. Says Cambridge University physicist Richard Friend, "[Such] molecular semiconductors have now been built up as very credible materials for technologists." He adds that such work will speed industry's quest for all-plastic displays. "The level of interest is of an entirely different order than it was 2 years ago."