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Zener Diodes like all other diodes use a semiconductor P-N junction. The focus of this topic is actually more on the semiconductor characteristics than the diode it self. Semiconductors are not conductors and do not exhibit the same behavior than conductors. In this section free electron band and conductive band are used synonymously and Fermi level is a quantum mechanical term I can roughly define in semiconductor physics for a basic understanding as the kinetic energy of the electron in the highest quantum level.

Semiconductors do not have electrons in the conduction band. All electrons are bound in covalent bonds and their valance bands are nearly filled under normal condition. It is required for an external energy to increase the Fermi level of the electrons in the valance energy band to jump the energy gap (forbidden band) into the conduction band. Then the semiconductor becomes conductive. Thermal energy is one type of energy that can raise the Fermi level of the fermions (particularly electrons in this case) in the valance electron band to jump the energy gap into the conduction band. This will increase the probability for an electron to find it self in the conduction band resulting in the semiconductor to become more conductive. In this case we will see that the contribution that thermal energy makes to increase conductivity by raising the Fermi level is much more than the contribution thermal energy makes to increase resistance, by making the atoms vibrate more, increasing collusions between electrons. This cause the semiconductor to form NTC (negative temperature coefficients), The higher the temperature the better it conducts within normal boundaries.

With conductors, they already have electrons in conductive bands and by adding energy to it will not make that much difference in the amount of free electrons (conductivity). This cause a very small contribution to increasing conductivity. But the contribution to increase resistances are far greater due to the vibration of the atom causing more collisions between electrons. This give conductors a PTC (positive temperature coefficient) in normal conditions and cause the electrical resistance to increase when temperature increases.

One can also take note that not only silicon Si and germanium Ge are semiconductors, but carbon only appear to be a conductor because it conducts electricity and often misunderstood on "school science" level. It is actually a semiconductor and also have NTC (negative temperature coefficients).

Graphite can conduct electricity due to the vast electron delocalization within the carbon layers, a phenomenon called aromaticity. These valence electrons are free to move, so are able to conduct electricity. However, the electricity is only conducted within the plane of the layers.

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