The semiconductor transistors on the CPU chip itself will not wear out according to any normal circumstances. There are however a whole batch of things that could lead to failure of the transistors from other non-normal circumstamces. Here is a list of some of those factors.

1) Exposing the transistors to EMP (intense electro magnetic pulse such as may come from a nuclear blast). This can permanently alter the crystalline structure of the semiconductor leaving it as a non functional transistor, a short circuit or a lump of slag.

2) Exposing the thin oxide area of the transistor to high ESD (electro static discharge). This can cause a hole to be made in the oxide which would allow the gate to short to either the drain or source diffusions on the chip.

3) Exposing an I/O pin of the chip to a voltage over the Vcc or below the GND pin. This condition can casue the normal reverse bias that is expected between an N (or P) region of the chip and an adjacent P (or N) region. With forward bias the thus formed diode can conduct current into a part of the chip that is not a normal circuit path. All too ofetn this current finds its way to the opposite supply voltage through yet another forward biased P/N junction. If you happen to be unlucky the two pairs of junctions form a PNPN stack that acts like an SCR and will latch it self on. High current flow can lead to gradual or almost instant failure of the region around this un-intended SCR.

4) Latent holes in the oxide layer of the chip that insulates the metal layers on the top of the chip from the underlying conductive silicon diffusions may happen to lie under a metal run. These holes are there from an imperfection in how the crystal structure of the oxide was formed. As a chip operates the metal run may have a voltage level that is different than the voltage level of the under diffusion. This voltage difference sets up a electromagnetic force across this hole. It is possible for part of the metal to be pulled through the hole and create a short. This process, known as metal migration, can happen fast or slow. A chip could fail after years of operation due to this failure.

5) Note that ESD pulses can casue oxide holes too. These may not lead to immediate failure but could create a path for metal migration that leads to a future failure. THIS IS WHY ESD HANDLING PRECAUTIONS ARE SO IMPORTANT IN A ELECTRONICS ASSEMBLY AND TEST OPERATION. If ESD exists the stuff you are building and testing may look good today but may fail several months or years later in the field.

6) Exposing the Vcc to GND pin pair of a part to an applied voltage level that goes beyond the vendors specified maximum level. Excess voltage can stress the oxide insulation layers on a chip, it can cause the normally reverse biased tub and guard ring junctions of a chip to experience reverse breakdown and it can cause excess current to flow through transistors that get boased on harder than normal due to the increased supply level.

7) Exposing the chip and its transistors to excess heat can lead to accelerated metal migration, shifted transistor threshold voltages and general degrading of the crystal structure of the semiconductor.

Do we need more....?

Sometimes I wonder how it is even possible to make a complex chip like a microcontroller and keep it working considering all the things that could go wrong.

Michael Karas