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What do you mean by zener breakdown and avalanche breakdown?
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Answer Posted / gyanendu
"Answer:
"'Zener diode' and 'avalanche diode' are terms often used
interchangeably, with the former much more common. Both
refer to breakdown of a diode under reverse bias.
Specifically, when a diode is reverse biased, very little
current flows, and the diode is to a first order
approximation an open circuit. As the reverse voltage is
increased, though, a point is reached where there is a
dramatic increase in current. Equivalently, there is a
dramatic reduction in the dynamic resistance (slope of the
V-I curve) that can be as low as 1- 2  in this region.ÝThis
voltage is called the reverse breakdown voltage and it is
fairly independent of the reverse current flowing.ÝThis
property makes it ideal as a voltage reference.Ý
"Avalanche breakdown is caused by impact ionization of
electron-hole pairs.ÝWhile very little current flows under
reverse bias conditions, some current does flow.ÝThe
electric field in the depletion region of a diode can be
very high. Electron/holes that enter the depletion region
undergo a tremendous acceleration.Ý As these accelerated
carriers collide with the atoms they can knock electrons
from their bonds, creating additional electron/hole pairs
and thus additional current.ÝAs these secondary carriers are
swept into the depletion region, they too are accelerated
and the process repeats itself.ÝThis is akin to an avalanche
where a small disturbance causes a whole mountainside of
snow to come crashing down.Ý The efficiency of the avalanche
effect is characterized by a so-called multiplication factor
M that depends on the reverse voltage (Equation 1).
Equation 1: Multiplication Factor
"Here n is in the range 2 - 6, V is the applied (reverse)
voltage, and Vbr is the breakdown voltage.ÝThis is an
empirical relationship, as are many of the relationships
used to describe both Zener and avalanche breakdown.Ý
"Avalanche breakdown occurs in lightly-doped pn-junctions
where the depletion region is comparatively long.ÝThe doping
density controls the breakdown voltage.ÝThe temperature
coefficient of the avalanche mechanism is positive.ÝThat is,
as the temperature increases, so does the reverse breakdown
voltage.ÝThe magnitude of the temperature coefficient also
increases with increasing breakdown voltage. For example,
the temperature coefficient of a 8.2 V diode is in the range
3 - 6 mV/K while the temperature coefficient of an 18 V
diode is in the range ofÝ 12 - 18 mV/K.
"Zener breakdown occurs in heavily doped pn-junctions.ÝThe
heavy doping makes the depletion layer extremely thin. So
thin, in fact,Ýcarriers canít accelerate enough to cause
impact ionization.ÝWith the depletion layer so thin,
however, quantum mechanical tunnelingÝ through the layer
occurs causing current to flow.ÝThe temperature coefficient
of the Zener mechanism is negativeóthe breakdown voltage for
a particular diode decreases with increasing
temperature.ÝHowever, the temperature coefficient is
essentially independent of the rated breakdown voltage, and
on the order ofÝ -3 mV/K.
"In a 'Zener' diode either or both breakdown mechanisms may
be present. At low doping levels and higher voltages the
avalanche mechanism dominates while at heavy doping levels
and lower voltages the Zener mechanism dominates.ÝAt a
certain doping level and around 6 V for Si, both mechanism
are present with temperature coefficients that just cancel.
It is possible to make Zener diodes with quite small
temperature coefficients.Ý
"Neither Zener nor avalanche breakdown are inherently
destructive in that the crystal lattice is damaged.ÝHowever,
the heat generated by the large current flowing can cause
damage, so either the current must be limited and/or
adequate heat sinking must be supplied."
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