Transistors wear out

It turns out that transistors eventually wear out, much like mechanical components of a machine do. There was an interesting article about this in this month's IEEE Spectrum magazine. This was "An Odometer for CPUs" by John Keane and Chris Kim. There's an on-line version of this article here.

It turns out that there are actually three different ways in which transistors wear out: hot-carrier injection, bias temperature instability and oxide breakdown. Here's how Keane and Kim described these:

Over time, charge carriers (electrons for negative, or n-channel, MOSFETs; holes for positive, or p-channel, MOSFETs) with a little more energy than the average will stray out of the conductive channel between the source and drain and get trapped in the insulating dielectric. This process, called hot-carrier injection, eventually builds up electric charge within the dielectric layer, increasing the voltage needed to turn the transistor on. As this threshold voltage increases, the transistor switches more and more slowly.

There's a second mechanism that can also trap charge in the dielectric, and it doesn't require any current to flow between the source and drain. Whenever you apply voltage to the gate, a phenomenon called bias temperature instability can cause a buildup of charge in the dielectric, along with other subtle problems. After that gate voltage is removed, though, some of this effect spontaneously disappears. This recovery occurs within a few tens of microseconds, making it difficult to observe during routine experiments, where you stress the transistor but measure the resulting effects only after the stress is removed.

Yet another aging mechanism comes into play when a voltage applied to the gate creates electrically active defects, known as traps, within the dielectric. If they become too numerous, these charge traps can join and form an outright short circuit between the gate and the current channel. This kind of failure is called oxide breakdown, or more verbosely, time-dependent dielectric breakdown. Unlike the other aging mechanisms, which cause a gradual decline in performance, the breakdown of the dielectric can lead to the catastrophic failure of the transistor, causing the circuit it's in to malfunction.

So even if Moore's law stops making your computers obsolete after a few years, it looks like you'll still have to replace them after not too long because their transistors will actually wear out.   

And when I read this article I had to wonder how transistors wearing out could affect side-channel attacks on encryption hardware. It's certainly possible that a piece of encryption hardware could be fairly resistant to side-channel attacks when it's new but gradually become vulnerable to them as it ages.

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