3-4
energy difference across this gap determines whether a solid material will act as a conductor, a
semiconductor, or an insulator.
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, as shown in figure 3-2, view A.
Normally, a semiconductor has no electrons at the conduction band energy level. The energy provided by
room temperature heat, however, 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.
View B of figure 3-2 is an energy diagram of a reverse-biased Zener diode. The energy bands of the
P and N materials are naturally at different levels, but 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 3-2B.-Energy diagram for Zener diode.
The second theory of reverse breakdown effect in diodes is known as AVALANCHE breakdown and
occurs at reverse voltages beyond 5 volts. This type of breakdown diode has a depletion region that is
deliberately made narrower than the depletion region in the normal PN-junction diode, but thicker than
that in the Zener-effect diode. The thicker depletion region is achieved by decreasing the doping level
from the level used in Zener-effect diodes. The breakdown is at a higher voltage because of the higher
