Others have already mentioned one failure mechanism that causes transistor degradation over time: electromigration. Other important aging mechanisms are negativ

Others have already mentioned one failure mechanism that causes transistor degradation over time: electromigration. Other important aging mechanisms are negative-bias temperature instability (NBTI) and hot carrier injection (HCI). I've seem papers claim the dual of NBTI - PBTI - is now an issue in the newest process nodes. This seems to be a nice overview of aging effects: http://spectrum.ieee.org/semiconductors/processors/transistor-aging http://spectrum.ieee.org/semiconductors/processors/transisto....