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There are two basic types of semiconductors; the intrinsic and the extrinsic. The material comprising an intrinsic semiconductor is in a generally pure state. The extrinsic semiconductor can be further categorized as either n-type or p-type. This is one to which impurities have been added to produce a desired state. N-type and p-type semiconductors are extrinsic semiconductors to which different impurities have been added, and consequently have different conductive properties.
A semiconductor is usually a crystalline solid in which conductivity due to electron flow is between that of a metal and an insulator. Intrinsic semiconductors are such materials with little or no impurity, silicon being the most widely used. The atomic lattice structure of silicon crystals is made up of perfect covalent bonds, which means there are few free electrons to move around. The crystal is almost an insulator. As temperatures rise above absolute zero, the likelihood of inducing electron flow in the material increases.
This effect can be greatly increased by introducing impurities into the lattice structure that make a greater number of free electrons available. The process of adding certain impurities to semiconductors is referred to as doping. The impurity added is termed the dopant. The amount of dopant added to an intrinsic semiconductor proportionally changes its level of conductivity. Extrinsic semiconductors are the products of the doping process.
Dopants are termed as either acceptors or donors, and change the charge carrier concentrations of a semiconductor. There are two types of charge carriers in semiconductors; a free electron and the hole where the electron used to be in an atom's valence band. The electron is a negative charge carrier, and the hole is considered to be positive charge carrier of the same magnitude. Donor dopants have more valence band electrons than the material it replaces, allowing more free electrons. Acceptor dopants have fewer valence band electrons than the material it replaces, creating more holes.
N-type semiconductors are extrinsic semiconductors in which donor dopants have been used. An increase in negative electron charge carriers results. Negative charge carriers are called the majority carrier in the n-type, while positive charge carriers are called the minority.
P-type semiconductors are the result of using acceptor dopants. As the covalent bonds of the lattice reform, holes are left in the valence bands of the surrounding material. The increase in holes increases the concentration of positive charge carriers. The majority carrier for the p-type would be positive and the minority negative.
By doping, semiconductors can be produced with different and complementary conductive properties. An important application of this is the p-n junction, where p-type and n-type semiconductors are brought into close contact. One effect of the junction is to permit the holes and the electrons to combine, producing light. This is a light emitting diode (LED). The p-n junction also forms a diode where electricity can flow in one direction through the junction but not in the other, a requirement for digital electronics.
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