Electronic Devices
10th ed.
Chapter 3
Special-Purpose
Diodes
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Electronic Devices
10th ed.
Objectives:
◆ Describe the characteristics of a zener diode and analyze
its operation
◆Apply a zener diode in voltage regulation
◆ Describe the varactor diode characteristic and analyze its
operation
◆ Discuss the characteristics, operation, and applications of
LEDs, quantum dots, and photodiodes
◆ Explain the basic operation of a solar cell
◆ Discuss the basic characteristics of several types of diodes
◆ Troubleshoot zener diode regulators
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Electronic Devices
The Zener Diode
The zener diode is designed to operate in the reverse
breakdown region.
I
F
Cathode (K)
Breakdown
VR
Anode (A)
VZ
VF
Reversebreakdown
region
Symbol
IR
Characteristic curve
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Electronic Devices
The Zener Diode
The zener diode is designed to operate in the reverse
breakdown region.
I
F
Cathode (K)
Anode (A)
Symbol
Ideally, the reverse
breakdown has a
constant breakdown
voltage. This makes it
useful as a voltage
reference, which is its
primary application.
Breakdown
VR
VZ
VF
Reversebreakdown
region
IR
Characteristic curve
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Electronic Devices
The Zener Diode
+
The zener impedance, ZZ, is the ratio of a
change in voltage in the breakdown region to
the corresponding change in current:
ZZ =
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VZ
I Z
ZZ
+
–
VZ
–
Practical model
Electronic Devices
The Zener Diode
+
The zener impedance, ZZ, is the ratio of a
change in voltage in the breakdown region to
the corresponding change in current:
ZZ =
VZ
I Z
ZZ
+
–
VZ
–
Practical model
What is the zener impedance if the zener diode voltage changes from
4.79 V to 4.94 V when the current changes from 5.00 mA to 10.0 mA?
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Electronic Devices
The Zener Diode
+
The zener impedance, ZZ, is the ratio of a
change in voltage in the breakdown region to
the corresponding change in current:
ZZ =
VZ
I Z
ZZ
+
–
VZ
–
Practical model
What is the zener impedance if the zener diode voltage changes from
4.79 V to 4.94 V when the current changes from 5.00 mA to 10.0 mA?
ZZ =
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VZ 0.15 V
=
=
I Z 5.0 mA
Electronic Devices
The Zener Diode
+
The zener impedance, ZZ, is the ratio of a
change in voltage in the breakdown region to
the corresponding change in current:
ZZ =
VZ
I Z
ZZ
+
–
VZ
–
Practical model
What is the zener impedance if the zener diode voltage changes from
4.79 V to 4.94 V when the current changes from 5.00 mA to 10.0 mA?
ZZ =
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VZ 0.15 V
=
= 30 W
I Z 5.0 mA
Electronic Devices
The Zener Diode
The temperature coefficient of a zener diode can be
specified as the percent change in zener voltage for each
degree Celsius change in temperature:
 VZ
TC = 
 VZ

 T

where TC has units of %/oC.
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Electronic Devices
The Zener Diode
The temperature coefficient of a zener diode can be
specified as the percent change in zener voltage for each
degree Celsius change in temperature:
 VZ
TC = 
 VZ

 T

where TC has units of %/oC.
Alternatively, it can be specified in terms of change in
voltage per degree Celsius change in temperature.
TC =
VZ
T
where TC has units of mV/oC.
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Electronic Devices
The Zener Diode
The temperature coefficient can be positive or negative,
depending on the zener voltage. Above 5.6 V, zeners
generally have a positive temperature coefficient; below
about 5.6 V, they have a negative temperature coefficient.
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Electronic Devices
The Zener Diode - Example
The temperature coefficient can be positive or negative,
depending on the zener voltage. Above 5.6 V, zeners
generally have a positive temperature coefficient; below
about 5.6 V, they have a negative temperature coefficient.
A 1N756 is an 8.2 V zener diode (8.2 V at 25o C) with a positive
temperature coefficient of 5.4 mV/oC. What is the output voltage if
the temperature rises to 50o C?
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Electronic Devices
The Zener Diode - Example
The temperature coefficient can be positive or negative,
depending on the zener voltage. Above 5.6 V, zeners
generally have a positive temperature coefficient; below
about 5.6 V, they have a negative temperature coefficient.
A 1N756 is an 8.2 V zener diode (8.2 V at 25o C) with a positive
temperature coefficient of 5.4 mV/oC. What is the output voltage if
the temperature rises to 50o C?
VZ = TC  T = ( 5.4 mV ) ( 25o C ) = 189 mV
VZ = 8.2 V + 0.189 V =
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Electronic Devices
The Zener Diode - Example
The temperature coefficient can be positive or negative,
depending on the zener voltage. Above 5.6 V, zeners
generally have a positive temperature coefficient; below
about 5.6 V, they have a negative temperature coefficient.
A 1N756 is an 8.2 V zener diode (8.2 V at 25o C) with a positive
temperature coefficient of 5.4 mV/oC. What is the output voltage if
the temperature rises to 50o C?
VZ = TC  T = ( 5.4 mV ) ( 25o C ) = 189 mV
VZ = 8.2 V + 0.189 V = 8.389 V
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Electronic Devices
The Zener Diode - Application
In low current applications, a zener diode can be used as a
basic regulator.
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Electronic Devices
The Zener Diode - Application
In low current applications, a zener diode can be used as a
basic regulator.
R
A 1N756 (8.2 V at 25o C) is used as an
8.2 V regulator in the circuit shown.
What is the smallest load resistor that
can be used before losing regulation?
Assume an ideal zener diode model.
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1.0 kW
+
VIN
18 V –
1N756
8.2 V
RL
Electronic Devices
The Zener Diode - Application
In low current applications, a zener diode can be used as a
basic regulator.
R
A 1N756 (8.2 V at 25o C) is used as an
8.2 V regulator in the circuit shown.
What is the smallest load resistor that
can be used before losing regulation?
Assume an ideal zener diode model.
The no load zener current is I NL =
1.0 kW
+
VIN
18 V –
1N756
8.2 V
VIN − VZ 18 V − 8.2 V
=
= 9.8 mA
R
1.0 kW
This is the maximum load current in regulation. Therefore, RL =
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8.2 V
=
9.8 mA
RL
Electronic Devices
The Zener Diode - Application
In low current applications, a zener diode can be used as a
basic regulator.
R
A 1N756 (8.2 V at 25o C) is used as an
8.2 V regulator in the circuit shown.
What is the smallest load resistor that
can be used before losing regulation?
Assume an ideal zener diode model.
The no load zener current is I NL =
1.0 kW
+
VIN
18 V –
1N756
8.2 V
VIN − VZ 18 V − 8.2 V
=
= 9.8 mA
R
1.0 kW
This is the maximum load current in regulation. Therefore, RL =
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RL
8.2 V
= 837 W
9.8 mA
Electronic Devices
Zener Diode Application
Zeners are embedded in three-terminal regulators to establish
a reference voltage. These circuits are capable of much larger
load currents than the basic zener regulator shown previously.
Control
element
VIN
Ref
VIN
Voltage
regulator
Reference ground
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VOUT
Error
amplifier
VOUT
Feedback
element
Electronic Devices
Zener Diode Application
Zeners can also be used as limiters. The back-to-back zeners
in this circuit limit the output to the breakdown voltage plus
one diode drop.
R
Vin
D1
D2
+VZ1 + 0.7 V
0
–VZ1 – 0.7 V
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Electronic Devices
Zener Diode Application
Zeners can also be used as limiters. The back-to-back zeners
in this circuit limit the output to the breakdown voltage plus
one diode drop.
R
What are the maximum
positive and negative
voltages if the zener
breakdown voltage is 5.6 V?
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Vin
D1
D2
+VZ1 + 0.7 V
0
–VZ1 – 0.7 V
Electronic Devices
Zener Diode Application
Zeners can also be used as limiters. The back-to-back zeners
in this circuit limit the output to the breakdown voltage plus
one diode drop.
R
What are the maximum
positive and negative
voltages if the zener
breakdown voltage is 5.6 V?
± 6.3 V
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Vin
D1
D2
+VZ1 + 0.7 V
0
–VZ1 – 0.7 V
Electronic Devices
Varactor Diodes
A varactor diode is a special purpose diode operated in
reverse-bias to form a voltage-controlled capacitor. The
width of the depletion region increases with reverse-bias.
p
n
Plate
Plate
Dielectric
– VBIAS +
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Electronic Devices
Varactor Diodes
A varactor diode is a special purpose diode operated in
reverse-bias to form a voltage-controlled capacitor. The
width of the depletion region increases with reverse-bias.
If the depletion widens, does
the capacitance increase or
decrease?
p
n
Plate
Plate
Dielectric
– VBIAS +
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Electronic Devices
Varactor Diodes
A varactor diode is a special purpose diode operated in
reverse-bias to form a voltage-controlled capacitor. The
width of the depletion region increases with reverse-bias.
If the depletion widens, does
the capacitance increase or
decrease?
A
Hint: C =
d
p
n
Plate
Plate
Dielectric
– VBIAS +
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Electronic Devices
Varactor Diodes
A varactor diode is a special purpose diode operated in
reverse-bias to form a voltage-controlled capacitor. The
width of the depletion region increases with reverse-bias.
If the depletion widens, does
the capacitance increase or
decrease?
A
Hint: C =
d
Notice that as the effective
plate separation widens, the
capacitance will decrease.
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p
n
Plate
Plate
Dielectric
– VBIAS +
Electronic Devices
Varactor Diodes
C T, diode capacitance (pF)
Capacitance tolerance range are the range of values of
capacitance for a given varactor. The data sheet will show
the minimum nominal and maximum values, which are
often plotted on a graph.
100
70
50
30
10
7
5
1N5148
1N5144
1N5139
3
1
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TA = 25 °C
f = 1 MHz
1.0
3.0 5.0 7.0 10
VR , reverse voltage (V)
30
50 60
Electronic Devices
Varactor Diodes
For example, you can
use this graph to read
the capacitance as a
function of reverse
voltage for various
diodes.
C T, diode capacitance (pF)
Capacitance tolerance range are the range of values of
capacitance for a given varactor. The data sheet will show
the minimum nominal and maximum values, which are
often plotted on a graph.
100
70
50
30
10
7
5
1N5148
1N5144
1N5139
3
1
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TA = 25 °C
f = 1 MHz
1.0
3.0 5.0 7.0 10
VR , reverse voltage (V)
30
50 60
Electronic Devices
Varactor Diodes
The capacitance ratio is the ratio of the diode’s capacitance
at the minimum reverse voltage (largest C) to the diode’s
capacitance at the maximum reverse voltage (smallest C).
Maximum Ratings (TC = 25°C unless otherwise noted)
Rating
Symbol
Value
Unit
Reverse voltage
VR
60
Volts
Forward current
IF
250
mA
RF power input*
Pin
5.0
Watts
Device dissipation @ TA = 25 °C
Derate above 25°C
PD
400
2.67
mW
mW/° C
Device dissipation @ TC = 25°C
Derate above 25°C
PC
2.0
13.3
Watts
mW/° C
TJ
+175
°C
T stg
–65 to +200
°C
Junction temperature
Storage temperature range
*The RF power input rating assumes that an adequate heat sink is provided
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Electronic Devices
Varactor Diodes
The capacitance ratio is the ratio of the diode’s capacitance
at the minimum reverse voltage (largest C) to the diode’s
capacitance at the maximum reverse voltage (smallest C).
Maximum Ratings (TC = 25°C unless otherwise noted)
Data sheets also include
parameters such as maximum
ratings for current, power
and temperature.
Rating
Symbol
Value
Unit
Reverse voltage
VR
60
Volts
Forward current
IF
250
mA
RF power input*
Pin
5.0
Watts
Device dissipation @ TA = 25 °C
Derate above 25°C
PD
400
2.67
mW
mW/° C
Device dissipation @ TC = 25°C
Derate above 25°C
PC
2.0
13.3
Watts
mW/° C
TJ
+175
°C
T stg
–65 to +200
°C
Junction temperature
Storage temperature range
*The RF power input rating assumes that an adequate heat sink is provided
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Electronic Devices
Varactor Diodes
Varactor diodes are used in tuning applications. The applied
voltage controls the capacitance and hence the resonant
frequency.
+
–
VBIAS
R2
C1
R1
R3
C2
Vin
Vout
L
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D
Electronic Devices
Varactor Diodes
Varactor diodes are used in tuning applications. The applied
voltage controls the capacitance and hence the resonant
frequency.
+
By varying R2, the
reverse bias on D is
changed. This changes
the capacitance, and
hence the resonant
frequency.
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–
VBIAS
R2
C1
R1
R3
C2
Vin
Vout
L
D
Electronic Devices
Diode Symbols
Some common diode symbols are shown:
Zener
Varactor
PIN
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Light-emitting
Laser
Tunnel
Photo
Schottky
Current-regulator
Electronic Devices
Selected Key Terms-1
Zener diode
Varactor
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Electronic Devices
Selected Key Terms-1
Zener diode A diode designed for limiting the voltage
across its terminals in reverse bias.
Varactor
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Electronic Devices
Selected Key Terms-1
Zener diode A diode designed for limiting the voltage
across its terminals in reverse bias.
Varactor A variable capacitance diode.
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