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Diode Construction
Diodes are made from semi-conductor materials like silicon and germanium.
Conduction in semi conductors is through two different mechanisms:
· Electron movement. Electrons are negative charge carriers. They move in an n-type semiconductor like electrons do in a wire.
· Holes. These are positive charge carriers. A hole is where there is a deficiency in electrons. The principal charge carriers in a p-type semiconductor are holes. When a voltage is applied, the hole moves about. The atoms do not move, but the deficiency transfers from atom to atom. The hole has a positive charge of +1 e (1.6 × 10-19 C)
When there is no voltage applied the electrons and holes drift about randomly. When a p.d. is applied, the holes drift to the negative and the electrons go to the positive. Where there are equal numbers of electrons and holes, i.e. in pure semi conductor materials, the semi conduction is called intrinsic. The current is very small, although if the material is heated, the conductivity increases. N-type materials are made by adding small amounts of impurities (doping), like phosphorus. In P-type materials, aluminium is the doping material.
Semi-conductor diodes are small, reliable, and robust. The silicon diode is particularly suited to high current applications. They can have high working temperatures, and can withstand reverse-biased voltages of several hundred volts.
However, if abused, diodes will fail. Like all semi-conductors they can go into thermal runaway if the current is too heavy. In semi-conductors the resistance falls with higher temperatures. Therefore the current gets bigger still, and the diode gets hotter, and so on… Also a high reverse voltage will destroy a diode completely.