A semiconductor is an electrical device that acts as
a conductor under certain conditions and as a
nonconductor under other conditions. The most
popular of all semiconductors is silicon. In its pure
state, silicon is neither a good conductor nor insulator.
But by processing silicon in the following ways, its
conductive or insulative properties can be adjusted to
suit just about any need. When a number of silicon
atoms are jammed together in crystalline (glasslike)
form, they form a covalent (sharing) bond. Therefore,
the electrons in the outer ring of one silicon atom join
with the outer ring of other silicon atoms, resulting in a
sharing of outer ring electrons between all of the
atoms. It can be seen in figure 1-4 that covalent sharing
gives each atom eight electrons in its outer orbit,
making the orbit complete. This makes the material an
insulator because it contains more than four electrons
in its outer orbit. When certain materials, such as
phosphorus, are added to the silicon crystal in highly
controlled amounts, the resultant mixture becomes a
conductor (fig. 1-5). This is because phosphorus,
which has five electrons in forming a covalent bond
with silicon (which has four electrons in its outer
shell), will yield one free electron per molecule, thus
making the material an electrical conductor. The
process of adding impurities to a semiconductor is
called doping. Any semiconductor material that is
doped to yield free electrons is called N-type material.
When boron, which has three electrons in its outer
ring, is used to dope the silicon crystal, the resultant
Figure 1-4.Covalent bonding of silicon.
Figure l-5.-Phosphorus-doped silicon.