What makes one metal a better conductor than another metal?

I know that , for instance, copper and silver have different resistivity/conductivity etc… but why? What makes silver a better conductor than copper (why does silver have a lower resistivity than copper)? Metals don’t have conduction/valance bands right? Or if they do they are overlapped and the vaccuum level is the same for all metals?

This question arises from metal stacking on HEMT gates. I’ve got an AlGaN/GaN HEMT with a 3 metal stack on the gate but from what I understand the Ni on the bottom is the only one that affects the Schottky barrier height, so why the stack? Why not just nickel?

I know there is a simple answer here but I can’t put it into words.

The VSEPR (Valence Shell Electron Pair Repulsion) theory describes how the electrons on the outer shell behave. The easier it is for these electrons to flow, the less resistivity a conductor has.

From here: http://en.wikipedia.org/wiki/Electrical_resistance

See also: http://en.wikipedia.org/wiki/Vespr

As mentioned above, it is the electron mobility that gives rise to good electrical conductivity. This also improves thermal conductivity, and the high electrical conductivity also makes the metal a good reflector of light. So metals that, when cleaned of oxides and polished, are very shiny are good conductors of heat and electrical current. Note that the high electron mobility tends to make good conductors oxidize readily, so they tend NOT to stay shiny when exposed to atmosphere.

I am not sure the above replies are very useful, e.g. to say that it is because of increased mobility only transfers the problem elsewhere.

Electrons drift mobility is limited due to their effective mass in the solid (which is different than their true mass), and scattering due to the atoms. The amount of each depends on such factors as crystal arrangement and phonon coupling (e.g. lattice vibrational effects). I am not a physicist, so could not tell you what particular factors make gold and silver so good, but someone might be able to.

You need to be looking at the Fermi surfaces of the substance. The charge carriers of a conductor have an excitation energy of zero, as opposed to a non-zero energy in insulators. Thus, the charge carriers are free at any finite temperature and allow for electrical and thermal conduction. The shape and size of the Fermi surface determine all sorts of properties (like effective mass and conductivity).

I’ll try to dig out my solid state textbook and see if I can find a good summary, if you’re interested.