METALS
Metals exist in structures as ions, in metallic structures, they exist as positive sites in a lattice held together by a sea of electrons given up from their outer shells. If a P.D. is applied across a metalic structure, then these free electrons flow with it to the state of lower potential energy.

n = number of electrons per unit volume. v = average speed of electrons. l = length of wire. A = crosssectional area of wire. e = charge on an electron. The number of electrons in the wire is equal to n A l and so the total charge in the length of wire is n A l e The time taken for those electrons to pass through A (distance / time) is equal to l / v Thus the currant I is equal to n A l e v / l I = n A e v The currant density j (currant per unit cross sectional area) is equal to n e v

SEMICONDUCTORS
Intrinsic semi-conductors (e.g. silicon, germanium) have covalent lattices, at any other temperature than 0 K, there are always some electrons that manage to escape the covalent bonds and leave behind a whole in the structure. If a p.d. is applied across the material them electrons are attracted towards the positive end and holes move towards the negative end. Extrinsic semi-conductors have other atoms mixed into their lattices (doping) so that they either have more electrons not involved in bonding, n-type or they have more holes p-type. To make a p-type extrinsic semiconductor out of silicon, for example, alumimium might be added, to make an n-type, phosphorus might be added.