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4.7 Special purpose diodes

• Rectifier diodes and small-signal diodes are used to allow current to flow in
only one direction.
• But there are number of diodes with different terminal characteristics and
areas of application other than rectification.
• Some special diodes will be discussed briefly.
– Zener diode,
– LED,
– Photodiode,
– Varactor diode,
– Tunnel diode

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Zener diode
• Zener diode is heavily doped semiconductor diode that operates in breakdown
region.
• Due to high doping, the reverse breakdown voltage is low.
• Zener diodes are available with zener voltage of 1.8 V to 200 V with power ratings
from ¼ W to 50 W.
• It is connected in reverse bias - cathode is positive with respect to anode.

IZ +
VZ
−

– In a schematic symbol z stands for zener.

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Characteristic curve of Zener diode iD

• Forward characteristic curve of zener diode is −VZ −VZK

similar to that of the normal diode and is not vD
− I ZK
shown.
• In breakdown region, the characteristic is Q − I ZT
ideally vertical → the voltage across the diode 1
( test current )
remains constant independent of the reverse Slope =
rz
current that is flowing through it.
− I ZM
– This property is used to regulate the voltage.
• Ideal model of zener diode is constant voltage
source whose value is zener voltage.
• Used as voltage reference element.
Ideal model
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Breakdown mechanism
• Breakdown is a phenomenon due to which high current flows in the reverse
biased diode.
• When breakdown occurs in a diode, the current increases significantly and
voltage remains constant.
• In normal diode, breakdown is avoided as it may damage the diode.
• The diodes which specifically operate in breakdown region are called
breakdown diodes. Eg., zener diode.
• In pn junction, breakdown occurs through two mechanisms.
– Zener breakdown or Zener effect
– Avalanche breakdown or Avalanche effect

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Zener effect d = 200 A

= 200  10−10 m
V =2V
• Zener breakdown occurs in heavily doped pn junction. V
E=
• Mechanism d
2
=
– As impurity is high, the depletion region is thin. 200  10 −10

= 1 10 V/m 8

– Small reverse voltage will develop strong electric field in the depletion region. = 110 V/cm 6

– This high electric field breaks covalent bonds of the silicon atom.
– Large number of carriers are generated in the depletion region.
– High current flows through the diode and voltage remains constant - breakdown voltage.
– This process is called a zener breakdown.
• In diodes with breakdown voltage < 4 V, breakdown occurs due to zener effect.
• When temperature is increased, the breakdown voltage of the diode reduces. So,
temperature coefficient of zener effect is negative.

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Avalanche effect A
A

A
A

• Avalanche breakdown occurs in lightly doped pn junction. High A

A

KE A
• Mechanism
– When high reverse voltage is applied across the diode, the depletion region widen and acquires
high electric field.
– The electrons that enter the depletion region are accelerated due to high electric field.
– The accelerated electrons collide with the atoms and break the covalent bonds generating new
electron-hole pairs (free carriers).
– These new carriers will also gain sufficient energy to create still another electron-hole pair.
– This cumulative process results in a large number of minority carriers and reverse current is
increased.
• In diodes with breakdown voltage > 6 V, breakdown occurs due to avalanche effect.
• If the temperature is increased, the breakdown voltage of the diode increases. So,
temperature coefficient of avalanche effect is positive.
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Difference between avalanche and zener effect

Avalanche breakdown Zener breakdown

• Occurs in lightly doped diode. • Occurs in heavily doped diode.
• Occurs due to collision of highly • Occurs due to strong electric field in
accelerated carrier with atom. the depletion region.
• Breakdown voltage is high (typically • Breakdown voltage is low (typically
> 6V). < 4V).
• Temperature coefficient is positive. • Temperature coefficient is negative.

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Light-Emitting Diode (LED)

• LED converts a forward current into a light.
• The light emitted is proportional to the forward current.
• Available in variety of colors - red, green, yellow, blue, orange, white, or infrared.
– Color of a LED is determined by the semiconductor material used.
• LEDs have a typical voltage drop from 1.5 V to 2.5 V for currents between 10 to 50
mA.

RS ID
ID
Anode Cathode +
VD
+ VD − VS
−
– In symbol, the arrows point outward.
• It is always connected in forward-bias.
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• Applications of LED
– Power indicator,
– Displays (7 segment display),
– Emergency light
– Infrared LED is used in burglar-alarm system, remote controls.
• Advantages
– Solid state light source,
– Fast on-off switching,
– Low power consumption,
– Extremely bright,
– Longer life,
– Low cost
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Photodiode
• Photodiode converts light signal into electrical signal.
• Photodiode has a clear window or lens to expose its pn junction to light.
Photodiode
• It is always connected in reverse-bias.
RS I
I
− +
+ p n +
VD I VS VD
RS
− VS −

– In schematic symbol, the inward arrows indicate the incoming light.

• When a reverse biased photo diode is exposed to the incident light, the reverse current
increases. This current is known as photocurrent.
• The photocurrent is proportional to the intensity of the incident light.
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Characteristic curves of photodiode

Reverse current- illumination curve Reverse voltage-reverse current curve
Reverse I  ( μA )
current
VT
400
50 40 30 20 10 0 V ( V )
Dark current

1000 fc
100
1500 fc
200
200
2000 fc
Dark 300
current
Illuminance 2500 fc
1000 2000 400
(fc)

• Reverse current is directly proportional to • The reverse current is almost independent on

illumination. the reverse bias voltage.

• The dark current is the current that will exist with no applied illumination.
• Light intensity is normally measured in lumens/ft2 (lm/ft2), footcandles (fc) or W/m2.

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• Application of photodiode
– Optical transmission and communication system.
– Photo detector in counting system – to count items in conveyor belt.
– It is widely used in burglar alarm systems.

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Varactor diode or varactor

• Varactor diode is a semiconductor voltage dependent variable capacitor.
• It provides a variable capacitance which may vary from about 2 to 100 pF depending
on the varactor diode considered.
• It is also known as voltage variable capacitor, varicap, or tuning diode.
• It is always connected in reverse bias.

Anode Cathode

• It utilizes the property of voltage dependent junction capacitance of the diode that
exists in the reverse biased pn junction.

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• The junction capacitance of diode is a function of reverse voltage as

C j ( 0) C ( pF )
Cj = C0
(1 + VR V0 )
m

where, V0 = barrier potential

C j ( 0 ) = capacitance at the zero bias condition
VR ( V )
m = grading coefficient (1 3 to 1 2)
– It shows that the capacitance decreases with increase in reverse voltage.
• Applications of Varactor diode
– Voltage tuning of LC resonant circuit
– Automatic frequency control device
– Self balancing bridge circuit
– Tuning of television receivers, FM receivers and other communication equipment.
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Tunnel diode
• Tunnel diode is a highly doped diode with breakdown voltage at 0 V.
• Tunnel diode is highly conductive semiconductor diode.
• The current conducts through tunneling.
• Tunneling
– The depletion region is too thin due to high doping. Tunnel diode
– Many carriers can penetrate through the junction even when they do not possess
enough energy to overcome the potential barrier.
– This phenomenon is called a tunneling and hence the diode is named tunnel diode.
• It is also called Esaki diode. IT
Anode Cathode
+ VT −
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• When reverse biased, current rises sharply with

Characteristic curve of tunnel increase in reverse voltage because breakdown
diode occurs at 0 V
– the diode acts as an excellent conductor
• The forward bias also produces immediate
conduction.
IT Negative resistance – The current reaches a maximum value IP (peak
region current) for the small diode voltage VP (peak
tunneling voltage).
Conventional
current IP forward bias – If the forward voltage is further increased, the current
current decreases to a minimum value IV (valley current) at
voltage VV (valley voltage).
IV
– For voltage greater than VV, the current starts to
VP VV increase exactly as a normal diode.
VT
• The region between the peak and valley points is
called a negative resistance region because an
increase in voltage produces a decrease in current.
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• Advantages of tunnel diode

– Low noise;
– low power;
– high speed;
– Simplicity

• Application
– High speed switch (order of nano-seconds)
– In microwave oscillator and amplifier due to small capacitance
– Relaxation oscillator circuit due to negative resistance

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Symbols of diodes

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4.8 DC power supply

• DC power supply converts high voltage ac into one or more low dc voltages.
IL

220 V ( rms ) + Voltage +

vS Rectifier Filter VO Load
50 Hz - regulator
−
Step down
transformer

Function of each block of the power supply.

• Power transformer: It steps down line voltage to required value and also provides electric
isolation.
• Rectifier: It converts the input sinusoidal voltage to a pulsating dc voltage.
• Filter: It removes or minimizes the ripples present in the rectified output.
• Voltage regulator: It stabilizes magnitude of dc output regardless of change in load current.
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Rectifier

• Rectifier is a circuit that converts sinusoidal ac signal into a pulsating dc signal.

– pulsating dc signal contains both dc and ac component.

• The diodes used for rectification purpose are called rectifier diodes.

• Rectifiers are generally classified into

– Half wave rectifier

– Full wave rectifier

• Center-tapped full wave rectifier

• Bridge full wave rectifier

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Half wave rectifier

D
vS vO
+ +
VS VP
vAC
ac vS RL vO t
t

T - -

• On the positive half cycle of secondary voltage, the +

D
+
diode turns on and current flows through the load. vac
AC
vS RL vO
– The output across the load is an exact replica of the iD
- -
applied signal. T

• On the negative half cycle, the diode turns off and the
output voltage drops to zero. D
- +
• It utilizes alternate half cycle of the input sinusoid and vAC
ac vS RL vO
produces the output voltage. iD = 0
+ -
T
• The output voltage is a half-wave rectified signal.
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Parameters of half wave rectifier

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Some definitions
• Rectification efficiency – a ratio of dc power delivered to load to an ac input power
from the transformer.

• Ripple factor – is a measure of an amount of ripple present in the waveform. It is

defined as the ratio of rms value of ripple to average value of the output signal.

• PIV rating – maximum reverse voltage that can withstand by the diode without
damaging it.
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Center tapped full wave rectifier

vS D1 vO
VS + VP
vS Center tap RL
t vAC - t
ac - v
+ +
vS O

-
T D2 D1
• The center tapped transformer provides two equal voltages across +
the two halves of the secondary with the polarities indicated. vS vO iD
vAC - - +
•
ac
During positive half cycle, the diode D1 is on and D2 is off. The + RL
vS
current flows through D1, RL and back to the center tap of the -
transformer. T D2

• During negative half cycle, the lower end of the secondary winding D1
becomes positive; the diode D1 is off and D2 conducts. The current -
flows through D2, RL and back to the center tap of the transformer. vS RL
vAC +
• The load voltage has same polarity in both cases. ac -
vS
- v +
O
iD
• The output voltage is a full-wave rectified signal. +
T D2
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Parameters of center tapped full wave rectifier

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Full-wave bridge rectifier

vS vO
VS D4 D1 VP
+
t - vO + t
vAC
ac vS
RL
T - D2 D3

• Bridge rectifier is the most popular rectifier. D4 D1

+
• During positive half cycle, the diodes D1 and D2 are forward - vO +
vAC
ac vS
biased and the current conducts through D1, RL, D2 and back to RL
T -
the transformer. D2 D3

• During negative half cycle, the diodes D3 and D4 conduct. The

current conducts through D3, RL, D4 and back to the D4 D1
-
transformer. - vO +
vAC
ac vS
• The load voltage has same polarity in both cases. +
RL
T D2 D3
• The output voltage is a full-wave rectified signal.
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Parameters of bridge rectifier

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Comparison among three rectifier circuits

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Rectifier with a filter capacitor

• The output of the rectifier is not purely dc but has an ac component (ripple) as
well.
• Capacitor is used to minimize the ripple of the rectified voltage.
• Filter capacitor is connected across the load.
D
+ vO T
VP
vAC
ac vS C RL vO

T - t

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vO T
D
+
VP
vAC
ac vS C RL vO
Vr ( p − p ) VDC
T -

t
Working principle: t = conduction interval
• During positive half cycle, diode conducts and capacitor is charged up to a maximum value of
VP .
• Beyond the peak, the diode cuts off and the capacitor discharge exponentially through the load
resistance until the input exceeds the capacitor voltage.
• Now, the diode turns on briefly and recharges the capacitor to the peak voltage and the
process repeats.
• The discharging time constant is made much greater than the discharging interval. So, the
capacitor will lose only a small part of its charge.
• The output of the filter circuit is almost a dc voltage with a small ripple caused by charging
and discharging of the capacitor.
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• The output dc load voltage will be

Vr ( p − p )
VDC = VP − ( approximate analysis )
2
• Assuming the ripple is a saw tooth wave, the rms of ripple voltage is given by
Vr ( p − p )
Vr ( rms ) = ( triangular wave assumption )
2 3

• Ripple factor

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Zener diode voltage regulator

• Voltage regulator is a circuit that provides a regulated dc voltage across the
load regardless of the voltage fluctuation in the source and current drawn by
the load.
• Zener diode can be used as a voltage regulator. i D

– Zener diode should be reverse-biased and must −VZ −VZK

operate in breakdown region. vD

− I ZK
– Breakdown operation is ensured by two conditions:
• VS > VZ Q − I ZT
• IZk < IZ < IZM ( test current )
1
Slope =
rz
– Zener diode is always connected across the load − I ZM
→ a shunt regulator

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Zener voltage regulation

• When the input voltage or load changes in a Zener voltage regulator, the current
flowing through the zener diode changes, keeping the output constant.
IS RS
IZ + IL +
VS VZ RL VL
• Zener diode provides two types of voltage regulation. − −
– Voltage regulation when input voltage changes - Line regulation

VS  I S  VL , I L = constant  I Z ( = I S − I L ) 

– Voltage regulation when load resistance changes - Load regulation

RL  I L  VL , I S = constant  I Z ( = I S − I L ) 
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How zener diode regulates the voltage? (Graphical analysis)

IS RS
• Consider that input voltage fluctuates between VS1 to VS2 IZ +
(VS2 > VS1). VS VZ
• Applying KVL, we get equation of load line: −
VS − VZ
IS =  IZ iD
RS
−VS 2 −VS 1 −VZ
– For VS1, maximum zener current that can flow is IZ1(max) vD
and maximum voltage that can appear acorss zener is VS1 . Q1
−I Z1
– For VS2, maximum zener current that can flow is IZ2(max) Q2 −IZ 2
and maximum voltage that can appear acorss zener is VS2.
− I Z 1( max )
• The load lines on the characteristic curve demonstrate that as
− I Z 2( max )
VS varies from VS1 to VS2 , the zener current changes from IZ1 to
IZ2 but the zener voltage remains almost equal to VZ .

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Zener diode model

IZ +
VZ
− Ideal model

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Analysis of the zener voltage regulator with load

• When load is connected across the zener, the thevenin’s voltage driving the zener diode is
less than input voltage.
• To ensure the breakdown operation, Vth > VZ .
I S RS
IZ + IL +
VS VZ RL VL
− −

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Chapter 2 complete.
Any question?

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