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UNIT-2
Semiconductor Diode
1
2.1 P-N JUNCTION:• PN-junction is formed by growing a single crystal of
Si or Ge, which is half P-type and half N-type.
Fig.1 PN Junction
2
2.2 Formation of Depletion Region
Fig.2 PN Junction with Depletion Region
3
Depletion region or space charge region.
barrier potential or junction potential.
The barrier potential for Si is 0.7V & for Ge is 0.3V
at 25°C.
4
2.3 Bias of P-N Junction:Bias of PN junction
Zero bias or no bias
Forward Bias
Reverse Bias
5
Zero or No Applied Bias (VD=0 V)
• ..No current in ckt.
6
P-N Junction with Forward Bias
Fig. 3 PN Junction with Forward Bias
7
•
•
•
•
•
Applied voltage opposes the contact potential.
Net barrier potential VB is reduced.
Diffusion current increases.
Drift current slightly decreases.
The forward biasing current is
I f  I dif  I dr
8
P-N Junction with Reverse Bias
Fig. 4 PN Junction with Reverse Bias
9
•
•
•
•
VB increase.
Idif due to majority carriers reduces to almost zero.
Idr slightly increases.
The net current Ir remains constant till breakdown,
hence called saturation current Is
• Reverse saturation current.
• It is in Si of the order of nA & in Ge it is of the
order of μA
10
2.4 PN JUNCTION DIODE:..also known as crystal diode
..two terminal of diode are cathode and anode
..unidirectional two terminal device
.. Used as circuit element
P
N
(a)
Anode
Short Ckt.
(c)
P
+
N
-
Cathode (b)
Rf
VT= 0.7V
(d)
Fig.1 (a) Circuit symbol (b) schematic symbol (c) Ideal FB diode
(d) Practical diode equivalent ckt.
11
2.5 V-I Characteristics of PN junction Diode:V-I Characteristic
Forward Characteristic
.. It offers negligible impedance
Reverse Characteristics
.. It offers high impedance
12
V-I Characteristics Curve of Si & Ge Diode
Forward Characteristics
Breakdown voltages
Knee or cut in voltage
Reverse Characteristics
13
Breakdown voltage
Knee voltage or cut-in-voltage
Maximum forward current
Peak inverse voltage (PIV)
14
2.6 Temperature Effect on V-I Characteristics
15
2.7 Summary of Biasing Conditions
16
2.8 Comparison of Si & Ge Diode:S.
No.
1.
2.
3.
Parameter
4.
5.
Material Used
Cut-in Voltage
Reverse Saturation
current
Effect of temp.
Breakdown voltage
6.
Application
Si Diode
Ge Diode
17
2.9 Diode Current Equation:The diode current equation can be given as,


I  I 0 eV / VT  1
….(1)
η is the constant, η=1 for Ge & η= 2 for Si
VT=kT/e = T/11,600 Volt equivalent of temperature
if the diode is reverse biased then the diode current is,

I  I0 e
V / VT

1
…….(2)
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If V>>VT
then the term
e V /VT  1
so, I   I 0
Two parameter I0 & VT are the temp. dependent.
Breakdown voltage , when Temp.
Reverse Saturation current , when temp.
19
2.10 DEPENDENCE OF I0 ON TEMPERATURE :The dependency of I0 on temperature is given as,
I 0 KT e
m
 EG /VT
Where K is the constant which is independent of temp.
m = 2 for Ge & 1.5 for Si
• Reverse saturation current doubles its value for every 10ºC
rise in temperature.
20
2.11 DIODE RESISTANCE:An ideal diode offer zero resistance in F.B. &
infinite resistance in R.B.
Diode resistance
d.c. Forward Resistance
or Static Resistance
.. Static resistance RF= VF/IF
a.c. Forward Resistance
or Dynamic Resistance
.. Dynamic resistance rF= ΔVF/ΔIF
Static or DC Resistance
22
Dynamic or AC Resistance
23
Summary Table for Resistance Level
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2.12 Ideal Diode:
• Ideally, a diode should have
RF = 0 and RR = 
V-I characteristics of an ideal diode
30
2.13 Linear Piece-wise or Approximate Circuit
Model of a Diode:
31
2.14 Simplified Circuit Model:
32
Summary of Diode Models:
33
2.15 DIODE CAPACITANCE:Diode Capacitance
Diffusion or Storage Capacitance
Depletion or Transition Capacit.
.. Occur in FB junction
.. The amount of stored charge
Represents the magnitude of
diffusion capacitance.
..
CD= τIF/ηVT
Here, CD.. Diffusion Capacitance
η is constant
VT is the volt equivalent of temp.
τ mean life time of carrier
IF is the forward current
.. The typical value for CD is 0.02μF
.. Occur in RB condition
.. Due to the +ve & -ve immobile
Ions acts as dielectric medium so P
& N acts as two plate of capacitor.
..
CT= K/V1/2
Here, CT.. transition Capacitance
K is constant
V is the applied voltage
34
Transition and diffusion capacitance versus applied bias for a silicon
diode.
35
2.16 Application of PN diode:1. Rectifier ckts.
2. Clipping and clamping ckt.
3. Voltage multiplier
4. log & antilog amplifier ckt using OP-AMP
5. Freewheel diode
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2.17 VARACTOR DIODE:…voltage variable capacitance also known as varicaps
and voltcaps.
…. Used in Reverse bias condition
… The barrier capacitance is not a constant but
varies with applied voltage.
..Used for balancing bridge, Tuning of any LC circuit
Fig. (a) Circuit Symbol
(Under Reverse bias)
Fig. (b) Circuit Model
(Under Reverse bias)
40
Fig. Varicap Characteristic
CT Vs VR
In term of applied reverse bias, the transition capacitance is given by,
41
42
2.18 Regulated Power Supply:-
Fig. D.C. Regulated Power Supply
Transformer
 Rectifier
Filter
 Regulator
44
2.19 RECTIFIERS:- A.c. to d.c. converter
Rectifiers
Half Wave Rectifier
1-Φ HWR
..Used to convert a.c voltage into
pulsating d.c. voltage
Full Wave Rectifier
3-Φ HWR
Bridge FWR
Center tap 1-Φ FWR
3Φ FWR
45
2.20 HALF WAVE RECTIFIER:-
Fig.2 Half wave Rectifier
46
47
Fig.3 Input & output voltage wave form of HWR
48
Performance Parameters of HWR:(a) Average Current or d.c. current:-
49
2Vm
Vdc 
 0.318Vm
2
so the value of o/p voltage is 31.8% of max. a.c.
i/p voltage,
the avg. or d.c. value of current is,
I avg
Vdc I m
 I dc 

 0.318I m
RL 
50
Total RMS Value :
51
RMS Value of AC Components :
52
(b) Ripple Factor:The a.c. component present in the o/p is called
the ripple.
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(c) Peak Inverse Voltage for HWR:…..for HWR the PIV is Vm
54
(d) Efficiency of HWR:The rectifier efficiency is expressed as,
55
The efficiency will be max. when RL  rF
So, ηmax .  0.406  40.6%
56
(e) Transformer Utilization Factor (TUF):TUF is the ratio of the d.c. power delivered to the load and
the a.c. Rating of the transformer secondary.
Vdc .I dc
D.C. power delivered to the load
TUF 

AC power rating of the transform er
Vrms .I rms
Assuming R L  rF for HWR then we get,
Vm  I m 
 π   π 
2 2
TUF 

 2
π
Vm
 I m 

2   2 
TUF  0.287  28.7%
Ideal value of TUF is 100% practicall y it should be  .
57
If we take rF into considerat ion then w e have,
Now,
0.287 RL
TUF 
RL  rF
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(f) Voltage Regulation:The variation of d.c. o/p voltage as a function of
d.c. load current is called voltage regulation.
Vno load - Vfull load
% Voltage Regulation 
100%
Vfull load
An ideal power supply has full load voltage equal
to the its no-load Voltage and hence zero %
regulation.
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2.21 Center Tapped Full Wave Rectifier:-
Fig.2.16 (a) & (b) Center Tapped FWR
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2.22 FULL WAVE BRIDGE RECTIFIER:-
Fig.2.17 (a) Full Wave Bridge Rectifier
 The problem of center tapped transformer is eliminated in
bridge rectifier.
 The diode in bridge rectifier is required to have PIV rating
of only Vm.
 Only disadvantage is that it requires four diodes.
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2.23 Performance Parameter of Full Wave Rectifier:-
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Total RMS Value :
64
Ripple Factor :
65
Rectification Efficiency :
66
Peak Inverse Voltage for center Tap & bridge
FWR:……for FWR the net PIV is 2Vm
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2.24 Comparison of Various Rectifier:Sr.
Property
No.
1.
No. of diode
2.
Transformer Req.
3.
Efficiency
4.
Ripple Factor
5.
PIV
6.
O/P Frequency
7.
RMS current
8.
DC current
9. Voltage Regulation
1-Ф
HWR
1
No
40.6%
1.21
Vm
fi
Im/2
Im/π
Good
Center-Tap Bridge
FWR
FWR
2
4
Yes
No
81.2%
81.2%
0.482
0.482
2Vm
Vm
2fi
2fi
Im/1.414 Im/1.414
2Im/π
2Im/π
Better
Good
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2.25 FILTER CIRCUIT:Filters circuits are used to remove the a.c.
component those are very undesirable in rectifier o/p.
 There are two ways to do it :
1. Ripples are bypassed around a the load by using
a shunt capacitor.
2. Ripples can be limited to a low value by a series
inductor.
 A filter circuit is generally a combination of
capacitors and inductors.
 Filtering action depends upon the fact that:
Capacitor allows ac only to pass.
Inductor allow dc only to pass.
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2.26 (a) Shunt Capacitor Filter:
71
(a) Initial charging of the capacitor (diode is forward-biased) happens only once
when power is turned on.
(b)The capacitor discharges through RL after peak of positive alternation when the
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diode is reverse-biased.
(c)The capacitor charges back to peak of input when the diode becomes forwardbiased
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OUTPUT:
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Operation of Capacitor Filter:
During the conduction period, the capacitor gets
charged and stores energy.
During the non-conduction period, the capacitor
discharges through the load resistance delivering
energy to it.
Capacitor gets charged to the peak value quickly
because charging time constant is almost zero.
Discharging time constant is quite large, because it
discharges through the load resistance.
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As XC << RL, the ripples are bypassed through the
capacitor and only dc component flows through the load
resistance.
The Ripple Factor (r) is an indication of the effectiveness
of the filter and is defined as
where Vr(pp) is the peak-to-peak ripple voltage
and VDC is the dc (average) value of the filter’s output voltage
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• The variable Vp(rect) is the unfiltered peak rectified voltage.
77
Q. Determine the ripple factor for the filtered bridge rectifier
with a load as indicated in Figure below
Solution:- The transformer turns ratio is n = 0.1
The peak primary voltage is
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2.27 CLIPPER CIRCUITS:
• A circuit that clips off or removes a portion of the input
signal without distorting the remaining part.
• A clipper can process any type of signal.
• It is also known as wave shaping circuit.
•
Clipper Circuit
Positive
Clipper
Series
.. Diode &
Load in series
Negative
Clipper
Shunt
…Diode &
load in parallel
Biased
Clipper
Combinational
Clipper
85
Positive Clipper:-
Input wave
Series +ve Clipper
Output wave
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The Input.
Series +ve Clipper
The Output.
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Input wave.
•
Shunt +ve Clipper
Output wave
The diode is put in parallel with the load.
Practical Aspects of Parallel Clipper:
• If we take into account the threshold voltage,
VT = 0.7 V, the clipping level is not zero, but 0.7 V.
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Negative Clipper:-
Input wave.
Series -ve Clipper
The transfer characteristics
Output wave
91
Input wave.
Series -ve Clipper
Output wave
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2.28 Biased Clippers:

Bias means applying a dc voltage to change the
dc level of a circuit.
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Guidelines to Solve:
•
•
•
•
Determine the transition level at which the diode
turns ON.
With diode ON, find relation between vo and vi.
Draw the transfer characteristic of the clipper.
Plot the waveshape of vo for given input.
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We find that
•
•
•
•
•
Diode is ON for vi > VB.
Therefore, vi(tr) = VB.
When diode is ON, vo = vi – VB.
When diode is OFF, vo = 0.
Plot the transfer characteristic of the clipper.
95
Now, draw the
output.
मंगलवार, 11 अप्रैल
2017
Clippers and Clampers-1
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Example 1
• Determine the output waveform for the clipper
circuit, if the input is a sinusoidal wave of peak
value 15 V.
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Solution:
• The direction of the diode suggests that it will be
ON for positive values of vi.
• At transition level,
vd  0 V and id  0 V; so that vo  id RL  0
Writing KVL, vi (tr)  3  0  vi (tr)  3V
मंगलवार, 11 अप्रैल 2017
Clippers and Clampers-1
98 98

After diode is ON,
vo  vi  15
99
Transfer
characteristic
Draw the output.
Clippers and Clampers-1
100
Example 2
• Determine the output waveform for the clipper
circuit of Example 1, if its input is as follows
101
Solution:
Problem is simpler. Only two levels :
vi = +15 V and vi = -5 V
Clippers and Clampers-1
102102
Now, you can draw the output.
Clippers and Clampers-1
103103
Note that
1. Total swing of vi is 15 – (-5) = 20 V.
2. Total swing of vo is 18 – 0 = 18 V.
3. Clipper circuit clipped off 2 V, and raised the dc level by 3
V.
104
Example 3
• Determine the output of the parallel biased clipper
for the given input.
105
Solution:
106
Now, draw
the transfer
characteristic
Now, draw the output
wave, and get credit.
107
Example 4
• Repeat Example 3, taking a silicon diode with
VT = 0.7 V, instead of an ideal diode.
Solution :
To determine transition level, we use the
condition,id  0 A at vd  0.7 V
VR  id RS  0 V
108
Applying KVL,
Vi (tr)  VT  VB  0
 Vi (tr)  VB  VT  4  0.7  3.3 V
For inputs less than 3.3 V (including negative values), the
diode is ON, and vo = 3.3 V
For inputs greater than 3.3 V, the diode is OFF, and
vo = vi
as shown in figure.
109
Draw the output,
Note that VT reduces Vi(tr) to 3.3 V from 4 V.
110
2.29 Combination Clippers:-
•
D1 clips off positive parts above the positive bias
level.
•
D2 clips off below negative level.
•
This circuit is called a combination clipper.
111
112
2.30 Applications of Combination Clippers
• If the input voltage is very large compared to the bias
level, the output signal is a SQUARE WAVE.
• Thus, the circuit can be used for wave-shaping.
• It can also be used in a completely different way, as a
limiter used to protect a sensitive circuit (e.g., OPAMP, Galvanometer).
• The diodes conduct only when something abnormal
happens.
Sensitive
Circuit
113
2.31Clampers (Electronic Circuits):
• Also called DC Restorers.
• It clamps (or holds, or ties) either the positive or the
negative peak of a signal to a definite level.
• The circuit has a capacitor, a diode, and a resistor.
• In addition, it may have a dc supply to introduce
additional shift.
• Time constant τ = RC is made much larger than T
(time period) of the signal.
• The capacitor does not discharge when diode is not
conducting.
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Positive Clamper:-
(a) The input.
•
(b) The circuit.
On first negative cycle, the diode turns ON. The capacitor starts
charging. At negative peak, the circuit is as shown in (c). The
capacitor charges to Vm.
115
(c)
(d)
Slightly beyond negative peak, the diode turns OFF. Capacitor does
not discharge much because of high RC. At positive peak, the circuit
is as shown in (d). The net output is shown in (e).
116
(e) The output.
The charged capacitor acts like a battery of Vm. This is the dc voltage
that is added to the signal, as seen in (e). The output sits on 0 V
level. The output is shifted in positive direction.
117
Negative Clamper:-
(a) Input Wave
•
•
(a) The circuit.
(b) The output
The diode is turned around. The capacitor voltage reverses,
and the circuit becomes negative clamper. It is clamped to zerovolt level, but always remaining below 0 V.
Memory Aid : The diode points in the direction of shift.
118
Note :
• The total swing of the output is the same as that of the input.
• A clamper can also have an added dc voltage. It is then called
biased clamper.
• Start the analysis of the circuit for that part of input, for which
diode is ON.
• Assume that the capacitor charges to voltage level decided by
the circuit.
• Assume that when diode is OFF, the capacitor does not
discharge.
119
Example 1
(a) The input.
•
(b) The circuit.
Is it a positive or negative clamper ?
• Ans. : Biased positive clamper.
120
Solution :
The time constant of the circuit,
  RC  100 kΩ 1μF  100 ms
The time period of the input signal,
1
1
T 
 1 ms
f 1 kHz

  T
•
Thus, the capacitor holds the charge when the diode is OFF.
•
We begin analysis with the period from T/2 to T; the diode is
ON.
• The circuit is as shown in (a)
121
(a)
•
•
•
•
The output is across R, but it is also across 3-V battery.
Hence, vo = 3 V, during this period.
Applying KVL, -15 + VC -3 = 0;
VC = 18 V.
For the period from T to 3T/2, the circuit is as shown in (b).
122
(b)
•
Applying KVL, vo = 5 + 18 = 23 V.
•
Thus, the output is as shown in (c).
•
Note that the output swing is also 20 V.
(c)
123
Example
124
2.32 VOLTAGE MULTIPLIER:
• A Voltage multiplier is that of circuit which produces an
O/P d.c. voltage whose value is multiple of peak a.c. I/P
voltage.
• It is a combination of two or more peak rectifier circuit.
•
Voltage Multiplier (it contain diode &
•
Capacitor)
Voltage
Doubler
Half-Wave
Voltage Doubler
Voltage
Tripler
Full Wave
Voltage Doubler
Voltage Quadr-upler
125
Half Voltage Doubler:
Fig. Half-wave voltage doubler
Fig. Double operation,showing each half-cycle of operation:
(a) positive half-cycle;
126
Fig. Double operation,showing each half-cycle of operation:
(b) negative half cycle.
127
Full Voltage Doubler:
Fig. Full-wave voltage doubler
128
Fig. Alternate half cycles of operation for full-wave voltage doubler.
129
2.33 ZENER DIODE:.. Two terminal device e.g. anode & cathode.
..Heavily doped P-N junct. so depletion layer is about 100A◦
..Operates in breakdown region.
.. Breakdown voltage can be set by controlling doping.
Cathode
+
IZ
_
VZ
rz
Anode
+
_ VZ
(a) Ckt. Symbol of
Zener diode
+
_ VZ
(b) Practical Equivalent (c) Approx. zener equiv.
ckt. of zener diode
ckt.
Here rz is zener dynamic resistance of zener diode so rz = ΔVz/ΔIz &
130
value of rz varies from few Ω to several hundred Ω.
2.34 Biasing of Zener Diode:-
Biasing of Zener Diode
Both process due to of VRB
Reverse Bias
Forward Bias
.. It is identical to
the ordinary P-N Zener Breakdown
Avalanche Breakdown
..Observed in zener diode
Junction diode.
..Observed in zener diode
having Vz >8V
having Vz between 5 to 8V
..Breakdown Voltage as
Temp
..Breakdown Voltage
as Temp
..VI characteristic with
..VI characteristic with zener Avalanche breakdown is
Breakdown is very sharp.
gradually increases.
.. Due to colliding
..due to Electric field
minority carrier 131
2.35 V-I Characteristics of Zener Diode:-
132
2.36 Zener Diode Application :1. Zener diode as a voltage regulator.
2. Used as a peak clipper in wave shaping circuit.
3. It is used as fixed reference voltage in transistor
biasing circuit.
4.Used for meter protection.
133
2.37 Zener Diode as a Voltage Regulator :..Used to maintain a constant o/p d.c. voltage
RS
IS
IZ
+
_
IL
RL
VS
Regulated
VL Supply
VZ
Fig. 1 Zener Diode Shunt Regulator
For operation of circuit, The necessary condition VS>VZ
The I/P current is calculated by,
VS = ISRS+VZ
So
IS = (VS-VZ)/RS
…….eq.(1)134
Here Vs is the unregulated I/P voltage, VZ is the Zener
voltage.
For practical zener diode, zener resistance rz is taken
into account then load voltage is given by,
VL=VZ+IZ.rz ......eq.(2)
if rz is neglected Then,
VL=VZ
The current through the load resistance is
IL=VL/RL
……eq.(3)
The I/P current is given by,
IS=IL+IZ
……eq.(4)
135
Case1.Regulation when I/P voltage is Varied:-
Fig. (a) I/P Voltage is Varied
Case2.Regulation when Load Resistance is Varied:-
Fig. (b) Load Resistance is Varied
136
DRAWBACK:Large power dissipated in series resistance.
Large power wastage.
Diode Parameters:1. Zener Voltage (VZ)
2. Maximum Power Dissipation (PZmax)
3. Maximum Current (IZmax)
4. Minimum Current (IZmin)
IZmin = 10 % of IZmax
137
138
139
• This voltage (82.5 V) is more than
VZ (= 60 V).
• Hence, diode is ‘ON’.
• Replace the diode by its equivalent.
140
141
142
143
This is more than VZ. Hence diode is ‘ON’. We now
calculate the minimum value of zener current.
144
145
When V1 becomes 120 V :
146
Problem:3
(a) For the network of fig. below determine the range of RL
and IL that will result in VRL being maintained at 10V.
(b) Determine the maximum wattage reading of the diode.
147
148
149
2.38 Zener Diode as Sinusoidal Regulation:
(a) The Input.
(b) 40 volt peak to peak sinusoidal regulator
150
(b) Circuit operation at Vi=10V
(c) The Output.
151
2.39 Zener as Square Wave Generator:-
(a) The Input.
(b) Square wave Generator
(c) The Output.
152
2.40 Schottky Diode:• Schottky diodes are high-current diodes used primarily in
high-frequency and fast-switching applications.
• It is also known as hot-carrier diodes.
• It is formed by joining a doped semiconductor region
(usually n-type) with a metal such as gold, silver, or
platinum.
(a) Simplified geometry of
Schottky diode.
(b) Circuit symbol
153
Forward bias V-I characteristic of Schottky and pn junction diode.
2.41 Light Emitting Diode (LED):• The light-emitting diode (LED) is a solid-state light source.
• A light-emitting diode (LED) is a diode that gives off
visible light when forward biased.
• LED are made by using elements like gallium, phosphorus
and arsenic.
(b) Symbol of LED
Forward bias LED.
155
LED voltage and current
Advantages of LED
(i) Low voltage
(ii) Longer life (more than 20 years)
(iii) Fast on-off switching
Applications of LED
(i) As a power indicator.
(ii) Seven-segment display.
156
2.41 Tunnel Diode:• A tunnel diode is a pn junction that exhibits negative
resistance between two values of forward voltage.
• doping of p and n regions much more heavily than in a
conventional diode.
• It is made with germanium or gallium arsenide.
• It is also known as Esaki diode.
(b) Circuit symbol of tunnel diode
157
• The movement of valence electrons from the valence
energy band to the conduction band with little or no
applied forward voltage is called tunneling.
• Valence electrons seem to tunnel through the forbidden
energy band.
• It useful in oscillator and microwave amplifier
applications.
158