Sunday, October 25, 2009


A light bulb, floating over someone's head, has become a universal icon for a flash of insight. But the history of the incandescent bulb also shows that a good idea is not enough. The success of a new gadget often hinges on its ability to survive the rigors of the real world.

An electric current causes many materials to glow…momentarily. Glowing is a natural consequence of the "red-hot" temperatures produced by the current. But another consequence is reaction with oxygen in the air that promptly burns up the would-be filament. Protecting it in an air-free glass bulb is a key step toward practical electric light.

The filament material is also critical. The recently reopened Thomas Edison National Historical Site in West Orange, New Jersey recounts Edison's 1870s search through thousands of candidates, many involving carbonized threads of various sorts, including exotic materials like bamboo. Much of this selection process aimed at increasing the lifetime beyond the 15 or so hours of the first "successes." (British inventor Joseph Swan devised a similar device in Britain around the same time.)

Some 25 years later, Hungarian and Croatian inventors introduced the tungsten filaments like those we we use today, which last longer while providing more light.

The tungsten-halogen lamp extends the improvement. Its white-hot filament delivers much more light in the visible part of the spectrum, but over time even tungsten evaporates at these temperatures. Small amounts of halogens like iodine or bromine in the bulb react with the evaporated tungsten. The resulting halide migrates back to the filament where the heat decomposes it, leaving the tungsten back where it started, while the halogen goes on to pick up other stray atoms of tungsten. The result is a brighter, more efficient bulb.

Situations like this, where performance is directly improved by increasing the lifetime, occur frequently, for example in semiconductor electronics. In one example that I encountered while working in integrated-circuit technology a decade ago, making a transistor shorter improves its speed, both by increasing the electric field and by decreasing the distance electrons have to traverse. But a few of the more-energetic electrons cause atomic rearrangements that build up over time to change the transistor's properties and render it useless. The shortness of many transistors, and thus their performance, was directly limited by the need to avoid these "hot-electron effects."

The study of processes that lead to eventual failure goes by the somewhat misleading name of reliability. Typically, after an initially high failure rate, called "infant mortality," a batch of devices settles into a steady, low failure rate over time until some accumulating damage leads to eventual wearing out of all of the remaining devices.

Experiments on many similar devices are required to get a complete picture of the different ways they fail. Researchers need to know not just the median lifetime but its statistical distribution, to place strict limits on the number of possible failures. The statistics are also needed to guarantee that a complex circuit with many devices will continue to function.

Reliability engineers also need to quickly measure degradation with accelerated testing, for example at elevated temperature. They then extrapolate those results back to the milder conditions of ordinary wear and tear. For example, if you've owned your computer for a few years, its transistors have already been around much longer than the prototypes that were used to vet the latest manufacturing changes.

Confidently extrapolating wear-out times requires deep and accurate models of subtle, microscopic degradation mechanisms. As a result, reliability involves many fascinating physical phenomena, as well as an appreciation of statistics and of the ways that devices are put to practical use.

And as it does for the light bulb, this understanding can improve performance just as profoundly as a new invention can.


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