Tubular Transistors

Are organic semiconductors doomed to remain slow? Transistors built of organic semiconductors, which show promise for applications such as large-area electronics—think wall-size TV screens—perform poorly compared with their silicon counterparts when it comes to speed. The sluggishness arises because organic semiconductors lack the exquisite crystalline order of silicon, so electrons inside the organic material bounce around in it instead of traveling in a relatively straight line.

Now a collaboration of engineers at the University of Illinois at Urbana-Champaign; Columbia University, in New York City; and Dupont, in Wilmington, Del., has found a way to make organic transistors better. The group seeded thin layers of organic semiconductors with conducting carbon nanotubes. The nanotubes make up only about 1 percent of the hybrid material, so it retains the physical robustness of a normal organic semiconductor. But the nanotubes produce crystalline high-conductivity regions distributed throughout the transistor.

Turning on the transistor connects the nanotube regions, through which the electrons can travel with less rebounding off the underlying atomic structure. The electrons, in effect, take a shorter path through the transistor. The decreased distance increases the device's transconductance—its ability to control current with applied gate voltages—which is directly related to the speed of the transistor. The group's published work demonstrates a 60-fold improvement in transconductance in sample transistors. Although the improvement is not yet enough for commercial applications, the group says further experimentation will bring those within reach.

Carbon Nanotubes–Semiconductor Networks for Organic Electronics: The Pickup Stick Transistor, by X.-Zao Bo et al., Applied Physics Letters, Vol. 86, 2 May 2005.