Okay, where do I begin......Let me begin by stating how a MOSFET transistor works. The MOSFET or Metal Oxide Semiconductor Field Effect Transistors are the what makes up a CPU. They are basically very small on/off switches (that whole 0s and 1s thing).
There are there are four basic structures in a MOSFET. They can be seen
here. Now current is created when carriers, sub atomic quantum particles, move from source region to the drain region. These carriers are present in all semiconductor devices, but are more heavily concentrated in the source and drain regions. This is done during the fabrication of the MOSFET.
The problem is, that there is no path for the current to travel from the source to the drain when the device is off in the device shown. This is done by creating a channel between the source and drain by changing the electrical properties of the substrate below the gate. This channel is controlled by the gate and the amount of voltage applied across it. (The explanation on how the channel is created can get pretty technical so I will leave it out for now. Will post if you are really interested). This is why you sometimes have to increase the voltage when you are overclocking. The transistors are switching so fast that a proper channel cannot be created with the given voltage. Impurities and defects present during fabrication can affect the ability of the MOSFET to create a current channel.
Now as the name MOSFET states, it is a field effect transistor. This means that it is controlled by an electric field. This electric field is created by applying a voltage across the gate. It is this electric field that creates the channel between the source and drain because of its ability to change the electrical properties of the substrate below the gate.
the first three letters in MOSFET stand for metal oxide semiconductor. If you look at the gate structure, the gate is made of metal (in todays semiconductor devices it is made of polysilicon, but as devices get smaller the properties of metal start becoming better again, so it may move back to metal) then below it is the dielectric or oxide, then the semiconductor itself. The oxide is where the damage occurs if you apply to much voltage across the drain. All atoms are effected by electric fields in such a way that the electron cloud that surround the nucleus of the atom start pulling away from the nucleus. This creates a dipole structure. As you apply more voltage you make the electric field acting across the atoms in the oxide stronger. Eventually you get to a point where you will completely rip the electron cloud away from the nucleus, and you have dielectric breakdown. This in effect has destroyed the oxide between the metal or polysilicon and the silicon substrate and you can no longer create the channel in the substrate that makes the MOSFET work.
There is a relationship between oxide thickness and gate length. As you shrink the size of the gate length you make this oxide layer thinner making it more likely to breakdown and destroy the capacitive properties of the MOS structure. This is why AMD and Intel reduce voltage everytime they release a new smaller process. It's not to run the CPU cooler, its a side effect of moving to smaller technology.
In an ideal world the relationship above would give you about a 2V maximum voltage that can be applied across the gate for a 0.13um process. Unfortunately there are non-idealites in the world that can reduce the ability for the dielectric/oxide in the MOS struture to stand up to higher electric fields. This reduces the amount of voltage the transistors on your P4 can take before the atoms in the oxide give way and this is why the people who apply to much voltage across when overclocking and are using relatively high voltage get SNDS.
I hope I explained this clear enough for understanding. I am trying to fit what I learned in about 10 weeks in my Semiconductor Device Physics II class into one post. Any specific questions feel free to ask. Maybe it would be easier to answer if you have more specific questions. This was a general explantion, as best as I could fit into one post.
