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Electronics
Principles & Applications
Fifth Edition
Charles A. Schuler
Chapter 3
Junction Diodes
©1999
Glencoe/McGraw-Hill
INTRODUCTION
• The PN Junction
• Characteristic Curves of Diodes
• Diode Lead Identification
• Diode Types and Applications
The P-side of a junction diode is doped with acceptor atoms.
The N-side of a junction diode is doped with donor atoms.
junction
P
Anode
N
Cathode
A silicon crystal
Schematic Symbol
Zero Bias
The electrons near the junction cross
over and fill the holes near the junction.
Depletion region
Having no carriers, the depletion region is an insulator.
Forward Bias
The carriers move toward the junction
and collapse the depletion region.
The diode is on.
Reverse Bias
The carriers move away from the junction.
The depletion region is reestablished
and the diode is off.
200
5 ohms
175
150
10 ohms
mA
125
100
75
20 ohms
50
25
0
0
0.5
1.0
1.5
Volts
Resistor volt-ampere characteristic curves
Forward current in mA
200
175
150
125
100
75
knee
50
25
0
0
0.5
1.0
1.5
Forward bias in volts
Silicon diode volt-ampere characteristic curve
Linearity
• The volt-ampere characteristic curve for
a resistor is a straight line (linear).
• A diode has a non-linear characteristic
curve.
• The barrier potential produces a knee in
the diode curve.
• The knee voltage is around 0.6 to 0.7 volts
for silicon diodes.
Forward current in mA
200
175
100 oC
150
125
25 oC
100
75
-50 oC
50
25
0
0
0.5
1.0
1.5
Forward bias in volts
The effect of temperature on the diode curve
breakdown
600
Reverse bias in Volts
400
200
0
20
40
60
80
Reverse
current
in mA
100
120
140
Silicon diode reverse bias characteristic curve
Cathode lead
Anode lead
Cathode lead
V
mA
Anode lead
The diode is forward biased by the ohmmeter.
Cathode lead
V
mA
Anode lead
The diode is reverse biased by the ohmmeter.
Silicon diode ohmmeter testing
• Low resistance in both directions: the
diode is shorted.
• High resistance in both directions: the
diode is open.
• Relatively low resistance in the reverse
direction: the diode is leaky.
• The ratio of reverse resistance to forward
resistance is > 1000: the diode is good.
Diode quiz
At zero bias, diodes show a depletion region
which acts as an________.
insulator
The depletion region is collapsed by applying
_________ bias.
forward
The depletion region is made wider by applying
________ bias.
reverse
A forward-biased diode has its anode________
with respect to its cathode.
positive
Diode forward voltage drop decreases as
temperature ________.
increases
Reverse bias in Volts
6
4
2
0
20
40
60
80
100
120
140
A zener diode is designed to break down and
conduct backwards at lower voltages.
Reverse
current
in mA
Reverse bias in Volts
4
6
2
0
20
40
60
V
I
80
Reverse
current
in mA
100
120
140
V
The voltage across a conducting
zener is relatively constant.
Using a zener diode as a voltage regulator
Unregulated Supply
Load
The load is in parallel with the zener and will see a
relatively constant voltage as long as the zener is conducting.
This circuit is called a clipper or limiter.
When VIN < 1.2 VPP
VIN
VOUT
VOUT is not clipped
The diodes do not conduct.
When VIN > 1.2 VPP
VIN
VOUT
+ 0.6 V
VOUT is clipped
The diodes conduct.
- 0.6 V
This circuit is called a clamp or dc restorer.
When VIN > 1.2 VPP
C is charged.
VIN
C
VOUT
dc
VOUT is clamped.
0.6 V
VPP
- 0.6 V
VOUT(dc) =
2
CEMF
The inductive kick
can cause damage.
Transient suppression diodes can be used with inductive loads.
CEMF
The coil discharges
through the diode and
there is no arc.
LED
As the electrons cross the junction, they
lose energy in the form of photons.
LED circuit
RS
Power
supply
VS
VD
LED
IS =
VS - VD
RS
The typical voltage drop for most LEDs is from 1.5 to 2.5 V.
RS
Power
supply
VS
Photodiode
Photodiodes are reverse biased and
conduct in the presence of light.
Input
Output
Input
Optocoupler
Output
Step-index multimode fiber
Input pulse
Output pulse
The combined shorter and longer path lengths
act to stretch the output pulse.
Step-index multimode fiber
Due to pulse stretching, high speed data transmission is not possible.
Input
Output
Single mode fiber
Input
Output
A single path means no pulse stretching and
high speed data transmission is possible.
Tuning diode
L
C
ZERO BIAS
C is maximum and fR is minimum.
Tuning diode
L
C
REVERSE BIAS
C is less and fR increases.
Tuning diode
L
C
MAX. REVERSE BIAS
C is minimum and fR is maximum.
Diode applications quiz
A circuit used to control the amplitude of a
signal is the ________.
clipper
A circuit used to add a dc component to a
signal is the ________.
clamp
A device containing an LED and a photodiode is the ________.
optocoupler
A tuning diode shows less capacitance as
reverse bias ________.
increases
The device that is often used to regulate
voltage is the ________ diode. zener
REVIEW
• The PN Junction
• Characteristic Curves of Diodes
• Diode Lead Identification
• Diode Types and Applications