Transistor Aging

Illustration: Daniel Hertzberg

You know when it’s time to get a new car. Your odometer is far into six digits, perhaps the engine is burning lots of oil, or the transmission is growling. Fixing all that might well cost quite a bit more than your ancient vehicle is worth.

But what about your microprocessor? Unlike automobiles, microprocessors don’t have convenient little gauges that reflect how much wear and tear they’ve endured. And wear they do-though you’ll probably never notice it. The degradation of their transistors over time leads slowly but surely to decreased switching speeds, and it can even result in outright circuit failures.

You generally don’t perceive this deterioration because semiconductor companies always play it very safe-they set the clock-speed rating of their microprocessors so conservatively that almost every one of their products will continue to operate flawlessly throughout its intended lifetime. That strategy works. But it’s kind of like never taking your Ferrari out of the slow lane because you’re concerned that its engine might throw a rod 10 years down the road.

Several different phenomena can degrade the transistors on chips. How those phenomena combine to diminish a chip’s functioning depends on such factors as the circuit arrangement of the aging transistors as well as the voltages and temperatures they’re exposed to. With all these variables, it’s difficult to predict how the peak performance of a given microprocessor will decline over time.

We and other researchers are trying to improve that situation. One critical aspect of the work we did at the University of Minnesota was to develop better ways to study the different physical mechanisms of transistor aging. Today semiconductor engineers measure those aging effects primarily by examining transistors one at a time, using microscopic electrodes to probe a silicon wafer. The necessary equipment can cost tens of millions of dollars, and probing transistors individually is arduous when you’re trying to gather many observations. Sometimes you can’t do those measurements well, no matter how much time you spend.

We need better techniques. And we need them sooner rather than later. Microprocessors now contain billions of transistors, sometimes operating at clock speeds in excess of 3 billion cycles per second. The blazingly fast clocks mean that the transistors are exposed to lots of heat, which accelerates their decline. Another worry is that there are precariously small voltage differences between supply levels and the threshold at which the transistors turn on. Also, various improvements in the way silicon logic is fabricated have introduced new concerns about degradation. And transistors scaled down to today’s tiny dimensions experience more variation than ever in their operating conditions, which in turn leads to great differences from one transistor to another in how fast they wear out.

With better ways to measure transistor aging, chipmakers could let their microprocessors run faster-appreciably faster-than they do now. In the future it might even be possible to use these techniques to build circuits into microprocessors that continuously measure the subtle effects of aging and adjust clock frequency or operating voltages so that the transistors, old or new, could always run at peak speeds.