'Noisy' transistors gets a rethink

There is a newly found flaw in our understanding of transistor noise, a phenomenon affecting the electronic on-off switch that makes computer circuits possible.

According to engineers at the National Institute of Standards and Technology (NIST) who discovered the problem, it will soon stand in the way of creating more efficient, lower-powered devices like mobile phones and pacemakers unless it is solved.

While exploring transistor behaviour, the team found evidence that a widely accepted model explaining errors caused by electronic 'noise' in the switches does not fit the facts.

A transistor must be made from highly purified materials to function. Defects in these materials, like rocks in a stream, can divert the flow of electricity and cause the device to malfunction.

This, in turn, makes it appear to fluctuate erratically between 'on' and 'off' states.

For decades, the engineering community has largely accepted a theoretical model that identifies these defects and helps guide designers’ efforts to mitigate them.

Those days are ending, says NIST’s Jason Campbell, who has studied the fluctuations between on-off states in progressively smaller transistors. The theory, known as the elastic tunnelling model, predicts that as transistors shrink, the fluctuations should correspondingly increase in frequency.

However, Campbell’s group at NIST has shown that even in nanometre-sized transistors, the fluctuation frequency remains the same.

“This implies that the theory explaining the effect must be wrong,” Campbell said. “The model was a good working theory when transistors were large, but our observations clearly indicate that it’s incorrect at the smaller nanoscale regimes where industry is headed.”

The findings have particular implications for the low-power transistors currently in demand in the latest high-tech consumer technology, such as laptop computers. Low-power transistors are coveted because using them on chips would allow devices to run longer on less power — think mobile phones that can run for a week on a single charge or pacemakers that operate for a decade without changing the battery.

But Campbell says that the fluctuations his group observed grew even more pronounced as the power decreased.

“This is a real bottleneck in our development of transistors for low-power applications,” he says. “We have to understand the problem before we can fix it — and troublingly, we don’t know what’s actually happening.”

Campbell credits NIST colleague KP Cheung for first noticing the possibility of trouble with the theory.