"If you look at the possible range of electrical conductivity of materials, you find ceramics and other materials with strongly held electrons at one end - the insulators - and metals, with their de-localized electrons that are capable of moving around easily through the crystal structure, at the other. Pure metals are highly conductive, with silver the very best, even better than copper. Semi-conductors are in between.

The conductivity of a metal decreases with temperature. This is because the extra thermal agitation of the metal atoms results in shorter distances that electrons can travel through the crystal before they get deflected, which reduces the average rate at which they can travel through the structure - conductivity - decreases. The hotter the solid metal gets, the more the conductivity decreases. Any other barrier that can shorten the mean free path of an electron in a metal will also reduce conductivity, such as the presence of substitutional or interstitial solutes added to the base matrix. So bronze, for example, has lower conductivity than pure copper.

Semiconductors have the opposite reaction to temperature and carefully chosen impurities than that of metals. Semiconductor conductivity increases with temperature. This is because semiconductors are prevented from conducting at lower temperatures by the existence of energy gaps between the valence bands that its electrons can occupy. When a semiconductor, say, silicon, is heated, the electrons gain enough energy to jump those gaps and the electrons are then able to move, leaving holes behind that can be filled by other electrons. By choosing appropriate 'dopants,' the conductivity can be tailored to the conditions that the engineer wants, conducting by the motion of both positive (holes) and negative (electron) charges.

Obviously, there is a lot more to conductivity than this, but that's a brief summary."