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What Are Extrinsic Semiconductors?

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  • Written By: Geisha A. Legazpi
  • Edited By: Allegra J. Lingo
  • Last Modified Date: 26 November 2016
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Extrinsic semiconductors are partially conductive and partially insulating materials that have been chemically altered to bear a non-neutral electrical charge. They are the building blocks of semiconductor devices. The production of extrinsic semiconductors follows a successful production of intrinsic semiconductors and their transformation into positive (P)-type or negative (N)-type semiconductors.

When silicon dioxide undergoes removal of oxygen atoms, extraction of pure silicon possible. This pure silicon, while in liquid form, easily reacts with oxygen to revert to a variation of ordinary sand. By using a special production environment, such as in a vacuum or a non-reacting gas, the silicon material has a chance of having high purity. Any undesirable traces of other elements and compounds are also separated to achieve pure silicon. Silicon melts at about 2,577°F (about 1,414°C), thus special equipment and technology are required toward producing extrinsic semiconductors.

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Pure silicon by itself has to be doped so that it does not permanently stay as an intrinsic semiconductor. Doping involves introducing additional controlled impurities into the intrinsic semiconductor while it is in liquid form. In the electronics industry, pure silicon functioning as an intrinsic semiconductor needs to be converted into an extrinsic semiconductor in order to be used. If it has solidified as intrinsic, it needs to be molten again to create an extrinsic semiconductor. Once the intrinsic semiconductor is in liquid form, creating a P-type or N-type semiconductor is the next choice, and with the right dopant elements or correct choice of controlled impurities, the intrinsic semiconductor becomes an extrinsic semiconductor or a doped semiconductor.

Extrinsic semiconductors are either N-type or P-type, depending on the dopant used. A dopant, such as boron, may have three electrons on the outer atom shell, or valence, to produce a P-type semiconductor. Those with five valence electrons, such as phosphorus, are used as dopants to produce an N-type semiconductor. Adding boron to molten pure silicon in a non-reacting environment makes it a P-type semiconductor, or an electron acceptor, while doping intrinsic silicon with phosphorus creates an N-type semiconductor, or an electron donor. One boron atom to as many as 10 million silicon atoms is the typical ratio of the amount of impurity in an intrinsic semiconductor.

A semiconductor plant delivers components with various combinations of extrinsic semiconductors. The two-terminal diode has a single P-N junction, or a joined P-type and N-type semiconductor. Very large-scale integration chips have thousands of junctions of P-type and N-type semiconductors.

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