_______________________________________________________________________ Special Devices
with the proper amount of energy added, electrons in the valence band may be elevated to
the conduction band energy level. To do this, the electrons must cross a gap that exists
between the valence band energy level and the conduction band energy level. This gap is
known as the FORBIDDEN ENERGY BAND or FORBIDDEN GAP. The energy
difference across this gap determines whether a solid material will act as a conductor, a
semiconductor, or an insulator.
3-9.
A conductor is a material in which the forbidden gap is so narrow that it can be
considered nonexistent. A semiconductor is a solid that contains a forbidden gap (see
Figure 3-2, view (A)). Normally, a semiconductor has no electrons at the conduction band
energy level. However, the energy provided by room temperature heat is enough energy to
overcome the binding force of a few valence electrons and to elevate them to the
conduction band energy level. The addition of impurities to the semiconductor material
increases both the number of free electrons in the conduction band and the number of
electrons in the valence band that can be elevated to the conduction band. Insulators are
materials in which the forbidden gap is so large that practically no electrons can be given
enough energy to cross the gap. Therefore, unless extremely large amounts of heat energy
are available, these materials will not conduct electricity.
3-10. Figure 3-2, view (B) shows an energy diagram of a reverse-biased Zener diode.
The energy bands of the P and N materials are naturally at different levels. However,
reverse bias causes the valence band of the P material to overlap the energy level of the
conduction band in the N material. Under this condition, the valence electrons of the P
material can cross the extremely thin junction region at the overlap point without acquiring
any additional energy. This action is called tunneling. When the breakdown point of the
PN junction is reached, large numbers of minority carriers "tunnel" across the junction to
form the current that occurs at breakdown. The tunneling phenomenon only takes place in
heavily doped diodes such as Zener diodes.
Figure 3-2. Energy Diagram for Zener Diode
3-11. The second theory of reverse breakdown effect in diodes is known as
AVALANCHE EFFECT and occurs at reverse voltages beyond 5 volts. This type of
breakdown diode has a depletion region that is deliberately made narrower than the
23 June 2005
TC 9-62
3-3
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