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Ask Dr. Knowledge

How long do transistors last?

May 23, 2011

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In the old days, television tubes could be taken to a drugstore for testing. A TV set could sometimes be fixed by determining which tubes were broken and replacing them. Now transistors have taken the place of tubes — how long do they last?

Old-fashioned vacuum tubes were basically little light bulbs with electrically heated filaments; electrons would boil off the filaments and collect on a metal plate inside the tube. The movement of the electrons inside the tube was facilitated by removing all the air, creating a vacuum. The air would otherwise inhibit the process. The flow of electrons could be controlled by electrical signals applied to grids placed between filament and plate, allowing tubes to be used as amplifiers or switches.

Tube-based devices could fail catastrophically during a mechanical shock — if the television was dropped, for example — or during a power surge. Tubes also aged, slowly losing effectiveness, because bits of the filament would evaporate as it was heated. Eventually the tube would burn out, much like an incandescent light bulb.

The vacuum tube worked only if the inside of the tube remained sealed. No seal is perfect, so air leaked into the tubes over time, interfering with the flow of electrons and degrading the metal elements. Some of the elements also could produce gases that would interfere.

Generally, transistors mimic the function of tubes but use semiconductors instead of a vacuum. Semiconductors are materials through which electricity flows easily. Transistors do not require heat to function, so they do not have any filaments to burn out or vacuum seals to leak. Properly packaged, they can take a lot of kicking around. Severe shock can damage them, though, as can strong surges of electricity. And they do age — just much more slowly than vacuum tubes. Aging effects are more important than many people realize.

Most computer chips could run faster, but when they do, they get hot, accelerating the aging process and often causing early failure. The life of a transistor is well beyond any reasonable expected lifetime of the product in which they are installed. Vacuum tubes were pretty much doomed to die before the rest of the television’s components. Old television sets had a dozen or so tubes, but most modern electronic devices have millions of transistors, often etched together on a tiny silicon chip. There is no way to pull out transistors and check them. When one fails, it is usually the end.

Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.