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Starting with a building block of silicon material it is possible to manufacture two basic types of semiconducting material, called N type and P type. When these two types are fused together a PN junction can be created forming the basis of a semiconductor diode.
The properties of pure silicon are such that it is not a very good conductor of electricity. These properties can be changed to make the silicon a more useful semiconductor by introducing an impurity that allows electricity to flow more readily. Such materials will change the structure of the silicon by either adding a free electron or by leaving a space (hole) that an electron can fill.
If arsenic is used then an extra electron becomes available for conduction. If boron is added then a hole becomes available for an electron to jump into.
The silicon with an extra electron becomes an N type material because it has an electron donated by the arsenic impurity.
The silicon with a deficient electron becomes a P type material because it has the ability to accept an electron provided by the boron impurity.
The N type semiconductor will have electrons as the majority carriers.
The P type semiconductor will have holes as the majority carriers.
When the two types of semiconductor are brought together then a PN junction is formed.
Depletion Layer - some of the electrons are attracted to the holes in the P type silicon and conversely some of the holes are attracted to the electrons in the N type silicon creating a depletion layer as depicted in Fig. 1.
The depletion layer - is analogous to having a small battery inside the device that must be overcome before any conduction will occur. For a silicon device the voltage needed to overcome this depletion layer is between 0.6 and 0.7 volts D.C. this is called forward biasing.
Reverse biasing - has the effect of increasing the depletion layer still further and no conduction will occur in the device. (see Fig. 3)
There is a point where increasing the reverse bias higher than the manufacturer’s recommendations will cause the diode to destruct and fail.