Analogue, Mixed Signal, LSI


Ballistic nanotransistor may lead to smaller and faster silicon chips

26 Jan 2000 Analogue, Mixed Signal, LSI

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 said to be 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 % 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. "In our device, we not only made the channel very short to minimise 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% 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 -200°C to reduce the scattering, or they used exotic materials. The Bell Labs nanotransistor has a 40-nanometer gate and its channel length is 25 nanometers.

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 %. However, when the researchers performed a computer simulation of a slightly thicker gate oxide - 1,6 nanometers - they predicted an 85% 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.





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