Tiny transistors sniff chemicals

One reason many teams of researchers are working to make electrical components from organic, or carbon-based, materials is these materials are inexpensive to manufacture.

Researchers from the University of Texas at Austin have found that the chemical sensing abilities of infinitesimally small transistors made from thin films of the organic crystal pentacene are quite different from those of larger transistors made from the same materials.

Electrical current flows from the source electrode of a transistor through its channel to the drain electrode. A transistor can be used as a sensor because the electrical flow can be affected by different conditions, including chemicals bonding to the semiconductor channel.

Nanoscale transistors make for more sensitive sensors than larger transistors because a change in electrical response that is due to the presence of a few molecules of a target substances is large enough to be detectable.

The researchers tested transistors that contained channel lengths ranging from 20 to 36,000 nanometers. A nanometer is one millionth of a millimeter.

The researchers found that the direction and amplitude, or strength, of sensing responses correlate to the length of the transistor channel and grain size of the pentacene crystals that make up the thin film.

Sensors made from the thin-film transistors have the potential to be very inexpensive. They could be manufactured using ink-jet printers.

The researchers are working on better understanding the interaction between various substances and the organic semiconductor layer, finding optimal semiconductor layers for various substances, and improving the durability of the organic semiconductor.

Such nanoscale sensors could be used practically within five years, according to the researchers.

The work appeared in the December 27, 2004 issue of Applied Physics Letters.