There are several other types of diodes beside the junction diode.
As the reverse voltage increases the diode can avalanche-breakdown
(zener breakdown).
This causes an increase in current in the reverse direction.
Zener breakdown occurs when the electric field near the junction
becomes large enough to excite valence electrons directly into the
conduction band.
Avalanche breakdown is when the minority carriers are
accelerated in the electric field near the junction to sufficient
energies that they can excite valence electrons through collisions.
Figure 4.6 shows the current-voltage characteristic of a
zener diode, its schematic symbol and equivalent circuit model in the
reverse-bias direction.
The best zener diodes have a breakdown voltage ( 
) of 6-7 V.
Figure 4.6: a) Current versus voltage of a zener diode,
b) schematic symbol for a zener diode and c) equivalent circuit model
of a zener diode in the reverse-bias direction.