_________________________________________________________________ Semiconductor Diodes
CONDUCTION PROCESS
1-48. As mentioned, energy can be added to electrons by applying heat. When enough
energy is absorbed by the valence electrons, it is possible for them to break some of their
covalent bonds. Once the bonds are broken, the electrons move to the conduction band
where they are capable of supporting electric current. When a voltage is applied to a crystal
containing these conduction band electrons, the electrons move through the crystal toward
the applied voltage. This movement of electrons in a semiconductor is referred to as
electron current flow.
1-49. There is still another type of current in a pure semiconductor. This current occurs
when a covalent bond is broken and a vacancy is left in the atom by the missing valence
electron. This vacancy is commonly referred to as a "hole." The hole is considered to have
a positive charge because its atom is deficient by one electron that causes the protons to
outnumber the electrons. As a result of this hole, a chain reaction begins when a nearby
electron breaks its own covalent bond to fill the hole, leaving another hole. Then another
electron breaks its bond to fill the previous hole, leaving still another hole. Each time an
electron in this process fills a hole, it enters into a covalent bond. Even though an electron
has moved from one covalent bond to another, the most important thing to remember is
that the hole is also moving. Therefore, since this process of conduction resembles the
movement of holes rather than electrons it is termed hole flow (short for hole current flow
or conduction by holes). Hole flow is very similar to electron flow except that the holes
move toward a negative potential and in an opposite direction to that of the electron. Since
hole flow results from the breaking of covalent bonds, which are at the valence band level,
then the electrons associated with this type of conduction contain only valence band energy
and must remain in the valence band. However, the electrons associated with electron flow
have conduction band energy and can therefore move throughout the crystal. A good
analogy of hole flow is the movement of a hole through a tube filled with balls (see Figure
1-9).
Figure 1-9. Analogy of Hole Flow
23 June 2005
TC 9-62
1-13
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