Engineering Principles V11

Clive W. Humphris

DIODE THEORY: The PN Junction Diode Characteristics.  

Semiconductor diodes commonly known as rectifiers allow a relatively high current to flow in one direction only, owing to a forward resistance of only a few ohms, whilst having a high resistance to current when connected in reverse.

To enable this action a semiconductor junction is formed by joining together two pieces of material, one doped as a p-type, having a deficiency of electrons called (+) holes (trivalent), the other n-type, having an excess of (-) electrons (pentavalent). At the point where p and n type materials come together, some electron/hole recombining takes place across the junction. This forms a neutral region with very few free electrons or holes, therefore very few current carriers.

Note the use of the term 'ion' to describe a positively or negatively charged atom. Negatively charged have an additional electron (donor atoms having five electrons in their valance shell) and positively charged atoms have a deficiency of electronics (where the donor impurity has only three in outer shell). Therefore n-type and p-type.

Because of this shortage of current carriers, the area either side of the PN junction acts as a poor insulator (semi-conduction), a bit like an ordinary resistor. By applying a forward bias voltage i.e. the p-type connection to battery (+), this region can be made to contract, thereby taking on a forward resistance of only a few ohms. This enables a high current to flow. Apply the battery in the opposite direction as a reverse bias and the junction expands forming an extremely high resistance.

The I, V curve shows a typical forward and reverse bias arrangement for a PN junction diode and will be explored fully as we proceed.

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