Reverse Bias characteristic of Zener Diode
Object: To draw the reverse bias characteristic of Zener diode and to determine Zener breakdown
voltage for three Zener diodes.
Apparatus used: Zener diode, voltmeter (0-20 volt), mili-ammeter, micro-ammeter, variable D.C.
source (0-20 volt).
Formula/theory:
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Zener diode is a heavily doped PN junction diode. Due to heavily doped, its depletion layer
is very thin and is order of micrometer. The forward bias characteristic of Zener diode is same as
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the normal PN junction diode but in reverse bias it has different characteristic.
Initially, a negligible constant current flow through the zener diode in its reverse bias but at
certain voltage, the current becomes abruptly large. This voltage is called as zener voltage. This
sudden and sharp increase in zener current is called as zener breakdown.
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The zener breakdown is caused by field emission of charge carriers. i.e. At zener voltage,
the field across the junction becomes very large due very thin depletion layer thickness. This
sufficiently strong electric field enables tunneling of electrons from the valence to the conduction
band of a semiconductor (or directly pulls the electrons form the semiconductor) leading to a large
number of free charge carriers. This sudden generation of carriers, rapidly increases the reverse
current.
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Circuit Diagram:
Fig 1: Zener Diode in RB
Procedure:
1. First of all, connect the circuit as shown in figure 1 for Zener first.
2. Now increase the voltage (VR) in intervals of 1volt and note the corresponding currents till
the current becomes out of scale. At the turning point of curve, measure current for small
interval of voltage also.
3. Repeat step 1-2 for other two zener diodes.
4. Plot the graph in VR and IR for all the three zener diodes.
6. Table for VR and IR of
third Zener diode
Sr.
VR
IR
No. (Volt) (mA)
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
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Observation:
1. Least count of voltmeter=……..volt
2. Least count of miliammeter =……..mA
3. Least count of micro-ammeter =…….μA
4. Table for VR and IR of
5. Table for VR and IR of
first Zener diode
second Zener diode
Sr.
VR
IR
Sr.
VR
IR
No. (Volt) (mA)
No. (Volt) (mA)
1.
1.
2.
2.
3.
3.
4.
4.
5.
5.
6.
6.
7.
7.
8.
8.
9.
9.
10.
10.
Result: The reverse bias characteristic of all the three Zener diodes are in graph1-3. The Zener
voltage for them are found as:
-6
-5
-4
-3
VR (volt)
0
-2
-1
VZ3=…….volts
0
-9
-8
-7
-6
-5 -4
0
-3
-1
-1
0
-2
0
-12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0
-1
-1
IR (mA)
-2
-2
VR (volt)
IR (mA)
VR (volt)
VZ2=……..volts,
-2
-3
-3
-3
-4
-4
-4
Graph-1
Graph-2
Graph-3
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Precautions:
1. The connection should be tight otherwise fluctuation in voltage and current will happen.
2. At the turning point of curve, more reading should be taken.
3. For plot of Graph, IR should be taken mA.
4. The reading should be in multiple of least count.
IR (mA)
VZ1=……..volts,