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Certain impurities can be added to semiconductors to increase their conductivity.
This process is called doping, and the semiconductor that contains impurities is called doped semiconductor.
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The Chemistry of the Electronics Industry
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Why do we use semiconductors in electronics?
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The main reason why semiconductor materials are so useful is that the
behavior of a semiconductor can be easily manipulated by the addition of
impurities, known as doping. Semiconductor conductivity can be
controlled by introduction of an electric field, by exposure to light,
and even pressure and heat; thus, semiconductors can make excellent
sensors. Current conduction in a semiconductor occurs via mobile or
"free" electrons and holes, collectively known as charge carriers.
Doping a semiconductor such as silicon with a small amount of impurity
atoms, such as phosphorus or boron, greatly increases the number of free
electrons or holes within the semiconductor.
http://en.wikipedia.org/wiki/Semiconductor_device
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A silicon atom has four valence electrons that can form four covalent
bonds with four other silicon atoms. If a dopant composed of atoms with
five valence electrons (e.g. antimony Sb) is added to a silicon crystal,
then only four of the electrons can pair up with electrons from silicon
atoms to form covalent bonds, leaving one free to move. This type of
semiconductor is called an N(negative)-type semiconductor. The following
is a model of an N-type semiconductor. The free electrons at the
beginning are marked yellow so that you can see them. Note that the
dopant concentration is greatly exaggerated (usually there is no more
than one dopant atom per ten thousand silicon atoms).
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If a dopant composed of atoms with three valence electrons (e.g. boron
B) is added to a silicon crystal, then one of the four covalent bonds is
short of an electron, leaving a hole. This type of semiconductor is
called a P(positive)-type semiconductor. The following is a model of a
P-type semiconductor. Note that the dopant concentration is greatly
exaggerated (usually there is no more than one dopant atom per ten
thousand silicon atoms).
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