Re:Physics? (Score 1)

I wouldn't say that the restivity is negative or positive. Rather, the electron mobility of elemental silicon increases as the temperature decreases. In doped silicon (such as you have in your CPU) or metallurgical grade silicon, electron mobility or hole mobilty increases as temperature decreases to a point, then decreases as the temperature drops further.
There's a bunch of fun graphs over at http://www.ioffe.rssi.ru/SVA/NSM/Semicond/Si/elect ric.html for you.