DRAM, as you said, basically consists of a storage capacitor and a transistor to access the voltage stored on that capacitor. Ideally, the charge stored on that capacitor would never decrease, but there are leakage components that allow the charge to bleed off. If enough charge bleeds off the capacitor, then the data cannot be recovered. In normal operation, this loss of data is avoided by periodically refreshing the charge in the capacitor. This is why it is called Dynamic RAM.
Decreasing the temperature does a few things:
- It increases the threshold voltages of MOSFETs and the forward voltage drop of diodes.
- It decreases the leakage component of MOSFETs and diodes
- It improves the on-state performance of the MOSFETs
Considering that the first two points directly reduce the leakage current seen by the transistors, it should be less of a surprise that the charge stored in a DRAM bit can last long enough for a careful reboot process. Once power is reapplied, the internal DRAM system will maintain the stored values.
These basic premises can be applied to many different circuits, such as microcontrollers or even discrete circuits, as long as there isn't an initialization on start-up. Many microcontrollers, for example, will reset several registers on start-up, whether the previous contents were preserved or not. Large memory arrays are not likely to be initialized, but control registers are much more likely to have a reset on start-up function.
If you increase the temperature of the die hot enough, you can create the opposite effect, of having the charge decay so rapidly that the data is erased before the refresh cycle can maintain the data. However, this should not happen over the specified temperature range. Heating the memory hot enough for the data to decay faster than the refresh cycle could also cause the circuit to slow down to the point where it couldn't maintain the specified memory timings, which would appear as a different error.
This is not related to bit-rot. Bit-rot is either the physical degradation of storage media (CD, magnetic tapes, punch cards) or an event causing the memory to become corrupted, such as an ion impact.
That appears to be just an extension of the saturation region if CB and BE are both forward biased:
http://en.wikipedia.org/wiki/Bipolar_junction_transistor#Regions_of_operation
It looks like your definition of saturation region is overly narrow.
If BE is reverse biased and BC is forward biased, then it still acts like a transistor, but because of the physical way bjt's are setup, you have much less gain than forward active.
Best Answer
As you may know, semiconductor devices are fabricated doping a very pure silicon (or other, less common, semiconductor materials) substrate using various kinds of ions. Doping different zones of the semiconductor with different types and concentrations of dopants produces the different kinds of semiconductor devices you are accustomed to (diodes, BJTs, FETs) and also (on integrated circuits) resistors and capacitors.
The doping ions give the semiconductor crystal its properties, but they are somewhat intruders in the regular intrinsic semiconductor lattice, since every thermodynamic system at a temperature above 0K tends, if left evolving, to a state of uniform concentration of chemical species. In other words, the ions tend to move away from their position in order to make their concentration in the crystal uniform. This phenomenon is called diffusion and it is contrasted by the forces of the chemical bonds that keep together the crystal.
Note that the bigger the amount of ionic diffusion, the more different regions of the chip lose their "identity ", i.e. their characteristics as electronic devices.
This effect is accelerated by high temperature because the thermal agitation tend to disrupt chemical bonds: ions with higher thermal energy diffuse more easily.
This phenomenon is always present, even at room temperature, but it's usually negligible. Nevertheless, ionic migration is not a linear effect, but an exponential one: so it increases dramatically with temperature. The max temperature listed by manufacturer is a threshold under which the manufacturer can guarantee that the device won't be damaged during the expected life of the part. Over that temperature, all bets are off and ionic migration and other temperature-related effects can actually damage the device in a relatively short time, i.e. the part could have its prospective operating life shortened.
Of course, if the max temp is 175°C and you run the part at 180°C it won't fail at once usually, but it will slowly degrade its performance. The higher the overtemperature, the quicker the degradation.
There are also other effects, though. At high temperatures the tiny wires connecting the chip to the package terminals (bond wires) could get damage from thermal stresses: the materials that make up the component have different thermal expansion coefficients, hence if the bond wire expands less than the surrounding material it may get damaged by excessive mechanical tension, for example. This same mechanism can damage the part at low temperatures (at -60°C you may even have cracks in the package, if you are unlucky enough).