Semi-conductors.
Diode:
This is composed of two different types of materials P-type and N-Type. P-type material is formed by doping Silicon (non-conductor) with a tri-valent atom such as Aluminium.
The structure is illustrated here, where each black dot represents an electron.
It must be remembered here that it is only the outer electrons which are shown in the diagrams.
Aluminium has only 3 electrons on its outer shell and these bond with the 4 neighbouring silicon atoms. This leave a deficiency of one electron and a hole is formed. This hole can accomodate an electron and the hole is the majority charge carrier in P-Type material.
N-Type material is formed when the silicon is doped with a pentavalent element such as Phosphorous. This has 5 electrons on its outer shell and so in bonding with the silicon it will have an "extra" electron which can carry the charge in conduction. The electron are the majority charge carriers in N-Type material. When N-Type and P-Type material are joined together they form a Junction Diode
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