Conductors, Insulators, and Semi-Conductors

Conductivity and resistivity are measures of the electrical properties which determine if something is a conductor, semi-conductor, or insulator.

A logarithmic scale is used to show conductivity in factors of 100, else too much space would be used up.

Silver, nickel, and gold, have a conductivity of 10^8 siemens per metre, and resistivity of 10^-8 ohm metres. They have conductance slightly lower than superconductors, which have no resistance at all. Metals are best conductors.

Steel and bronze have conductance of 10^6 siemens per metre, and resistance of 10^-6 ohm metres. Alloys tend to have lower conductance than pure metals. 

Pure germanium has conductivity 10^0 siemens per metre, and resistivity 10^0 ohm metres. It's a semi conductor, and conducts slightly less efficiently. 

Pure silicon has resistivity of 10^2 ohm metres, and conductivity 10^-2 siemens per metre.  It is a semi-conductor!

Pyrex glass has conductivity 10^-8 siemens per metre, and 10^8 ohm metres resistivity. It is an insulator. 

Perspex and lead glass has resistivity 10^12 ohm metres and conductivity 10^-12 siemens per metre. It is an insulator.

Measuring resistivity and conductivity in an insulator will require a large cross-sectional area, A, a small length, L, and a very very sensitive ammeter.

To measure resistivity and conductivity in a metal, a large length is needed, and a small cross-sectional area, A. Meanwhile, joule heating from the resistance should be accounted for, and drops in P.D. across the wire.

Semi conductors have conductivities between 0.01S-100Sm^-1. Germanium and silicon have been used historically, but also compounds such as gallium arsenide. 

Semi conductors can experience changes in electrical properties through doping. This is where impurities are added to intrinsic semi conductors, such as trivalent and pentavalent elements, which affects conductivity, and by combining layers of doped semi conductors.

Pentavalent elements provide 5 valence electrons, producing n-type conductors by providing extra electrons, donor impurity. Trivalent elements with 3 valence electrons produce p-type semi conductors by creating electron deficiency, acceptor impurity. Addition of pentavalant elements contributes more electrons, increasing conductivity. Phosporus may be added by diffusion of phosphine gas, (PH3).

Changes in voltage change electrical behaviour also, as semiconductor devices can actively modify signals applied to them. Since the 1950s the electronics industry has developed this behavior and created a range of devices including transistors, amplifiers, and complex computers. These become smaller and faster as semiconductor and metal elements are grown in situ on integrated circuits.