UKPCS Science: Conductors, semiconductors and insulators(2) #32

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Semiconductor materials

Elements that are used as semiconductors, such as silicon and germanium, have four outer shell electrons. This means that they can form four bonds with other identical atoms.

In a crystal of pure silicon each silicon atom is surrounded by four other silicon atoms. In this state the silicon will not conduct.

Pure silicon can be doped by tiny quantities of impurities by diffusing the impurities as a gas into the liquid semiconductor before it crystallizes. If an impurity element with five outer shell electrons, such as arsenic, is added to silicon in small quantities, (approximately one impurity atom to every one million silicon atoms), the impurity atoms will fit into the crystal structure but there will be one electron not bonded into the valence band of the crystal. This free electron is a charge carrier able to move within the conduction band.

This impurity causes the material to conduct and is called n-typesilicon.

If an impurity element with three outer shell electrons, such as Indium, is added to silicon in similar small quantities, the impurity atoms will fit into the crystal structure but there will be one electron missing.

These areas in the crystal where there is an electron missing are called (positive) holes. This impurity causes the material to conduct and is called p-type silicon.

n-type and p-type semiconductors do not have an overall electrical charge. The protons in the impurity atoms balance any increase or decrease in the number of electrons in the doped semiconductor.


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