This document explains the functioning and characteristics of zener and varactor diodes, including their operation under forward and reverse bias conditions. Zener diodes are primarily used for voltage regulation, where they maintain a constant output voltage despite variations in input voltage or load resistance. Varactor diodes function as variable capacitors, with their capacitance controlled by the reverse bias voltage.

1. By
Ms.J.Shiny Christobel
Assistant Professor/ ECE
Sri Ramakrishna Institute of Technology, Coimbatore
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2.  It is a properly doped Pn junction diode
 The arrowhead on a Zener diode symbol points in the
direction of forward current when the diode is forward
biased.
 The Zener diode is normally operated in reverse
breakdown and the current direction is then from anode to
cathode
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3.  The most basic Zener diode circuit consist of a single Zener
diode and a resister.
 The Zener diode provides the reference voltage, but a series
resistor must be in place to limit the current into the diode
otherwise a large amount of current would flow through it and
it could be destroyed.
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4.  When a Zener diode is forward biased, it operates
as a normal diode.
 In forward biased P side connected to +ve and N side
connected to –ve terminal of battery. In this case the
electrons and holes are swept across the junction an
large current flow through it.
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5.  In case of reverse biased current practically zero and
at certain voltage which called Zener voltage the
current increasessharply.
 Each Zener diode has breakdown rating which
specifies the max voltage that can be dropped
across it.
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7. The figure shows, theforward characteristics is same as
that of ordinary forward biasedjunction diode.
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8.  In reverse direction however there is a very small
leakage current between 0V an the Zener voltage –i.e.
tiny amount of current is able toflow.
 Then, when the voltage reaches the breakdown
voltage or Zener voltage (Vz), suddenly current flow
throughit.
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9. The Working Principle of zener diode lies in the cause of
breakdown for a diode in reverse biased condition.
Normally there are two types of breakdown.
 Zener Breakdown
 Avalanche Breakdown
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10.  The reverse bias on a crystal diode is increased, a critical
voltage, called Breakdown voltage is reached where the
reverse current increases sharply to a high value
 The breakdown region is the knee of the reverse
characteristic as shown in graph
 The breakdown voltage is sometimes called Zener
Voltage(Vz) and the sudden increase in current is known as
Zener Current
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12.  The breakdown or zener voltage depends upon the
amount of doping.
 If the diode is heavily doped, depletion layer will be
thin and consequently the breakdown of the junction
will occur at a lower reverse voltage
 A lightly doped diode has a higher breakdown voltage
 Zener diode is a crystal diode that is properly doped to
have a sharp breakdown voltage
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13.  This type of breakdown occurs at the reverse bias
voltage above 8V and higher.
 It occurs for lightly doped diode with large breakdown
voltage.
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14.  As minority charge carriers (electrons) flow across the
device. They tend to collide with the electrons in the
covalent bond and cause the covalent bond to disrupt. As
voltage increases, the kinetic energy (velocity) of the
electrons also increases. The covalent bonds are more
easily disrupted, causing an increase in electron hole
pairs. The avalanche breakdown voltage increases with
temperature.
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15. • This provides a constant voltage from a source.
• Zener diode of zener voltage VZ is reverse connected
across the load RL.
• The series resistance R absorbs the output voltage
fluctuations to maintain constant voltage across the
load.
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16. • The zener will maintain a constant voltage VZ(=E0)
across the load until input voltage does not fall below
VZ.
• The circuit should be designed in a manner such that
even if the input voltage Ei and load resistance RL
increases over a wide range, the load voltage E0
remains constant.
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17. Case 1:
• In the breakdown region zener diode is equivalent to
a battery VZ if the input voltage is increased.
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18.  Excess voltage is dropped across the series resistance R
which in turn increases the total value of current I.
 The zener will conduct the increase of current in I
when the load current remains constant. Hence the
output voltage E0 remains constant.
Case 2:
 If the input voltage is constant the load resistance RL
decreases which cause an increase in load current.
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19. Case 2:
• Voltage drop in a resistor cannot produce an extra
current from a source which will not change as the
zener is within its regulating range.
• Additional load current will come from a decrease in
zener current IZ.
• Voltagedrop across R=Ei-E0
• Current through R=IZ+IL
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20.  Varactor Diode is a reverse biased p-n junction
diode, whose capacitance can be varied
electrically.
 It is well known that the operation of the p-n
junction depends on the bias applied which can
be either forward or reverse in characteristic
 Under Reverse bias condition, the p-n junction
can be considered to be analogous to a capacitor
where the p and n layers represent the two plates
of the capacitor while the depletion region acts
as a Dielectric separating them.
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21.  Varactor diode as a capacitor
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Where, Cj is the total capacitance of the
junction.
ε is the permittivity of the semiconductor
material.
A is the cross-sectional area of the
junction.
d is the width of the depletion region

22.  The relationship between the capacitance and
the reverse bias voltage is given as
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Where, Cj is the capacitance of the varactor diode. C is the capacitance
of the varactor diode when unbiased.
K is the constant, often considered to be 1. Vb is the barrier potential.
VR is the applied reverse voltage. m is the material dependent constant.

23.  This indicates that the maximum operating frequency of the circuit is
dependent on the series resistance (Rs) and the diode capacitance, which can
be mathematically given as
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24.  Width of the depletion region d is inversely
proportional to C.
 This means that the junction capacitance of
the varactor diode decreases with an increase
in the depletion region width caused to due
to an increase in the reverse bias voltage (VR)
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25.  By controlling the level of doping, the varactor diodes can be
manufactured with a definite C-V curve. Depending on this,
varactor diodes can be classified into two types viz., abrupt
varactor diodes and hyper-abrupt varactor diodes.
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