CHAMPAIGN, Ill. - A new type of thin-film transistor developed at the University of Illinois could improve the resolution of flat-panel, liquid-crystal displays used in laptop computers. The transistor contains a novel "buried channel" that allows electrons to move faster, permitting much higher switching speeds.
"In conventional thin-film transistors, electrons travel near the semiconductor-insulator interface, where the silicon is strained and is of poor quality," said John R. Abelson, a U. of I. professor of materials science and engineering. "By creating a buried conducting channel, recessed about 50 angstroms away from the interface, we can increase the speed of the electrons and significantly enhance the performance of devices built with these transistors."
Flat-panel displays consist of hundreds of thousands of pixels, each controlled by a thin-film transistor. While the performance of these displays is adequate at present, future applications that require higher resolution (such as high-definition television or enhanced computer displays) will be limited by the speed at which the transistors can be turned on and off.
"Higher resolution means adding more pixels," Abelson said. "But as the number of pixels rises, there is less time to address each one and still produce a complete image at standard video rates."
Because the buried channel is placed in a region of higher-quality material, electrons can move through the device nearly twice as fast as they normally would, Abelson said. "This means it would take significantly less time to deliver the necessary charge to turn a pixel on or off, so you could address many more pixels in the same amount of time."
To fabricate the buried channel thin-film transistors, Abelson and graduate student Cory Weber use a technique called reactive magnetron sputtering. This method -- which uses a plasma to erode a silicon target and deposit a film -- provides precise control over layer composition and electronic properties.
"To create a step in the conduction band, and thus a buried channel, we vary the amount of hydrogen gas injected into the plasma while we deposit the amorphous silicon layer," Abelson said.
The sputtering technique also allows films to be deposited at much lower temperatures than currently possible with the plasma-enhanced chemical-vapor-deposition process used by industry.
"We have fabricated these transistors at a processing temperature of 125 degrees C," Abelson said. "This opens up the possibility of using lightweight and impact-resistant plastic substrates in place of the glass substrates currently employed. Production systems using reactive magnetron sputtering have recently been developed by the Intevac Corporation based on our results."
Abelson and Weber describe the fabrication and operation of the new transistor in the September issue of the Journal of Vacuum Science and Technology. A patent has been applied for.