Transistor aging (sometimes called silicon aging) is the process of silicon transistors developing flaws over time as they are used, degrading performance and reliability, and eventually failing altogether.
The main causes of transistor aging in MOSFETs are electromigration and charge trapping.
Electromigration is the movement of ions caused by momentum from the transfer of electrons in the conductor.
This results in degradation of the material, causing intermittent glitches that are very difficult to diagnose, and eventual failure.
Charge trapping is related to time-dependent gate oxide breakdown, and manifests as an increase in resistance and threshold voltage (the voltage needed for the transistor to conduct), and a decrease in drain current.