Originally posted by: pm
CPU's do age when in use. Manufacturer's rate them for an operating lifetime - how long depends on the manufacturer. If used continuously, eventually they will fail. Common mean time to fail ratings are on the order of 100,000 hours. 100k hours is over 10 years, so as far as the original question about turning off a laptop, I wouldn't worry about it.
They do not slow down in so far as performance is concerned except in highly unlikely circumstances. In general, they usually work until they fail - at which point, they do not work any more. In other words, a CPU that takes 10 seconds to calculate a spreadsheet in Excel when it's new, will still 10 seconds to calculate a spreadsheet in Excel after 10 years of use.
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In reality, the transistors gradually slow down as well. There are a couple of mechanisms that cause transistors to slow down over time: PMOS NBTI (Negative Bias Temperature Instability), and nMOS hot-e. A computer user doesn't notice this effect, however, because CPU's are digital synchronous devices. In other words, this effect of transistors slowing down over time can be considered to be "noise" that is ignored by the CPU because it has a clock and it's a digital device.
These two effects together are one of the reasons why overclocking works - manufacturers set a limit on the frequency that is less than the part is ever likely to slow down to within a given time period. So a CPU might come out of the fab at 2GHz and then after a year work at 1.98GHz, and after another year be limited to 1.96GHz, etc. Despite the fact that the CPU works fine at 2GHz at the time it's tested, it's "guardbanded" to a lower frequency so that it works correctly 10 years down the road. That guardband might, picking a random number that's easy to work with, be 10%. So rather than sell the CPU at 2GHz, it is marked and sold as 1.8GHz so that in 10 years it is still working. Higher voltages accelerate the two effects, and low temperatures accelerate hot-e while high temperatures accelerate NBTI.
This subject is one that is fairly esoteric - any references that I might provide will be to fairly unreadable electrical engineering papers. If you want more information, googling "hot carrier" "hot electron" "NBTI" "reliability" can find some sites with information (as well as, I note with some amusement, my postings here at Anandtech).
One good site that Google turned up for hot-e is: http://www.semiconfareast.com/hotcarriers.htm
