Ballistic Nanotransistor From Lucent's Bell Labs May Lead to Smaller and Faster Silicon Chips - Company Business and Marketing

Cambridge Telcom Report, Dec 13, 1999

Researchers at Lucent Technologies' Bell Labs have developed a method to significantly improve the flow of current in nanoscale transistors -- a characteristic that may help the semiconductor industry continue making smaller and faster silicon chips.

Dubbed a "ballistic nanotransistor" for its virtually unimpeded flow of current -- similar to a bullet whizzing through the air -- the device is roughly four times smaller than today's transistors.

In recent years, the semiconductor industry has increased the performance of chips by decreasing the size of their transistors, which increases their switching speed. However, one component of a transistor - its insulating layer - will limit the continued shrinkage because a short circuit will occur when it becomes too thin; the insulating layer lies between the transistor's gate, which turns the current on and off, and the channel, through which current flows.

To overcome the limitations posed by the insulating layer, the Bell Labs researchers decided to tackle another major factor that limits a transistor's speed: the resistance encountered by current as it flows through the channel.

In today's silicon-based transistors, only 35 percent of the input current flows, via the channel, from a transistor's "source" to its "drain;" the remainder scatters as it collides with the rough edges of the insulating layer.

"The electrons going through the channel are like a ball going through a pinball game," said Bell Labs researcher Greg Timp, who presented his research findings Monday at the International Electron Devices Meeting. "In our device, we not only made the channel very short to minimize the channel resistance, but we also removed nearly all the `pinball bumpers' by making the insulating layer smoother than it is in conventional transistors. This results in 85 percent of the current being transmitted from the source to the drain, which yields the ballistic transport."

Although other researchers have attained ballistic effects in nanotransistors, they needed to cool their devices to nearly minus 200 degrees Centigrade to reduce the scattering, or they used exotic materials.

"This is the first ballistic nanotransistor that operates at room temperature with conventional silicon technology," Timp said.

The Bell Labs nanotransistor has a 40-nanometer gate, which is roughly 2,000 times smaller than the width of a human hair, and its channel length is 25 nanometers.

There were several key elements involved in making a ballistic nanotransistor. Timp and his colleagues used an unconventional process, known as rapid thermal oxidation, to "grow" the insulating layer, or gate oxide, on the silicon wafer. This process adds oxygen to silicon at 1,000 degrees Centigrade for 10 seconds.

"This results in a smooth interface between the silicon wafer and the gate oxide," Timp said.

When Timp and his colleagues tested the new devices, they were surprised by a counterintuitive finding, which may have implications for the semiconductor industry. At first, the researchers tested nanotransistors with gate oxides that were only 1.3 nanometers thick, compared with today's average of 2.8 nanometers. The drive current efficiency was about 75 percent. However, when the researchers performed a computer simulation of a slightly thicker gate oxide -- 1.6-nanometers -- they predicted an 85 percent efficiency, which appeared odd because thicker gate oxides typically hinder current flow. Experimental results confirmed the prediction, which may ease the industry's need for making thinner gate oxide layers.

"It appears that electrons travel better when the gate oxide is slightly thicker because the electrons are not as attracted to the gate, which is directly above the gate oxide layer," Timp said.

Lucent Technologies, headquartered in Murray Hill, N.J., designs, builds and delivers a wide range of public and private networks, communications systems and software, data networking systems, business telephone systems and microelectronics components. Bell Labs is the research and development arm for the company. For more information on Lucent Technologies, visit the company's web site at http://www.lucent.com.