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SEMICONDUCTORS & DIODES CONDUCTORS, SEMICONDUCTORS AND INSULATORS i) For Conductors like Copper, Aluminum, Gold, and Silver their atomic structure contain large number of free electrons, there is no energy gap between valence band and conduction band as it is illustrated by fig. As soon as voltage is applied, electrons move through conductor. The energy gap is called as “forbidden gap”. When an electron of valence band gets extra it jumps into the conduction band. Actually electron becomes free from nuclear force when it goes into the conduction band. ii) Insulators sometimes called as bad conductors in which free electrons are absent. There is a large energy gap (5 to 10eV) between the conduction and the valence band. All the electrons of such material are tightly bounded by their nucleus; no free electron is available for carrying the current. Common examples are glass, mica, wood, air etc. iii) Semiconductors, whose conductivity lies in between conductors and insulators are
neither
conductors
nor
insulators
therefore
they
are
called
as
“Semiconductors”. INTRINSIC SEMICONDUCTORS Semiconductors in pure crystal form without any mixing of other substances are called as “intrinsic semiconductors”. Silicon and Germanium atoms are tetravalent atoms (tetra – means four) because there are four valence electrons in the last orbit as shown in fig.
SEMICONDUCTORS & DIODES
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