I had a college student build an overclocked PC using phase-change technology. (This is essentially an air-conditioning unit with the evaporator attached directly to the motherboard.) He said that cold temperature made the CPU more stable at higher clock speeds (5.5 GHz) by affecting how the atoms and electrons behave. My question is, is this true? If so, in what way does cold temperature affect semiconductors?
Temperature certainly affects semiconductors.
As someone who is not specialized in semiconductors, I can think of at least two microscopic effects at play here. The first is that at lower temperatures there are less phonons (quantized vibrations of the atomic lattice). The effect of phonons is to scatter the electrons and lower the conductance of the semiconductor. Thus at lower temperatures the conductance should be increased (as in the case of metals).
The second effect is that as the temperature is lowered, the fraction of electrons on high energy states is lowered (and on low energy states increased) thus potentially changing the amount of electrons electrons above/below the band gap (carrier concentration). The effect of this is more complex than the effect of phonons.
I am not really an expert on solid state physics, and I'm prepared to look an idiot here - but I don't think it's the low temperatures that help.
By overclocking a CPU 5.5 GHz you are almost doubling the power consumption and hence the dissipation. A large reduction in the temperature of the cold site of the heat sink helps it remove twice as much power while keeping the hot side (i.e. the CPU) at a reasonable temperature.
Resistance of semiconductors increases at lower temperatures and certainly once you get to cryogenic temps (below 100 K) you are very limited in the types of IC devices you can use and their performance.
I suspect that running at 5.5 GHz, putting out several hundred watts then even if the heat sink is in liquid nitrogen the CPU core is still near room temperature.
That is true. "Another reason that electronics has been operated at low temperatures is improved performance of the cooled electronics itself. Improvement upon cooling results from a combination of effects: in general, transistors (field-effect types) exhibit increased gain and speed and lower leakage; also parasitic resistances and capacitances in interconnections decrease, heat transfer improves, and many devices exhibit lower noise." (http://www.extremetemperatureelectronics.com/tutorial1.html)
Being too cold or too hot can cause problems in either case. Conductance at semiconductor junctions change with temperature. The junctions are more conductive at lower temperatures thus increasing switching speeds and less conductive at high temperatures thus decreasing switching speed.
It is this variation of switching speed that can cause different operating characteristics. Some parts of your circuit might start switching too fast or too slow for other parts. The idea of heating and cooling is to keep the entire circuit in the required temperature operating range. Also not to mention the obvious that if you are dumping 35 watts of power with nowhere for the power to go you are going to burn up your part.
This problem is first addressed at the design level by selecting parts with acceptable switching characteristics across the entire range of temperatures required for operation. Then it is the responsibility of the user to make sure that the design is kept in this temperature range during operation.