Doping of semiconductors

The P-N junction:

This occurs in the situation where a crystal has been doped such that half of it is n-type and the other half is p-type. The two halves have different Fermi levels (n-type’s is higher) and electrons flow from the n-type section to the p-type section to try and equalize the electron concentrations (fig. 9). This creates a positive charge on the n-type region and a negative charge on the p-type region which leads to an electric field pushing electrons back to the n-type region. Eventually a balance is reached (fig. 10).

If you apply an external potential difference to make the p-type region positive and the n-type region negative a continuous current can now flow. Electrons enter from the n-type electrode, travel through the conduction band of the n-type region, drop into the valence band of the p-type region, continue through the positive holes and then leave at the other electrode. Cannot flow the other way with a relatively low voltage as the electrons are unable to jump up to the n-type conduction band.