UNIT I

SEMICONDUCTOR DIODES AND THEIR APPLICATIONS

Text book: Robert L. Boylestad & Louis Nashelsky, “Electronic Devices and Circuit Theory”, 10 th
Edition, Pearson Education, 2013.

Contents:
• Semiconductor Diode
• Semiconductor Materials
• Depletion layer, V-I characteristics, Ideal and Practical
• Diode Resistance, Diode Capacitance, Transition and Diffusion Capacitance
• Zener Diodes:Breakdown mechanism (Zener and Avalanche)
• Diode Application
• Diode Equivalent Circuits (Series , Parallel and Series, Parallel Diode Configuration)
• Half and Full Wave rectification, Clippers, Clampers,
• Zener diode as shunt regulator, Voltage-Multiplier Circuits
• Special Purpose two terminal Devices
• Light-Emitting Diodes (LEDs), Liquid-Crystal Displays (LCDs)
• Varactor (Varicap) Diodes, Tunnel Diodes

Introduction:
Q. What is Electronics Engineering?
 The word electronics derives its name from the words ELECTRON and MECHANICS.
 The technology associated with low voltage, current and semiconductor solid state integrated
circuits, usually for transmission or processing of analog or digital data.

OR

 It is a field of science and engineering, which deals with electronic devices and their
utilization.
Q. What is Electronic Device?
 A device in which conduction takes place by the movement of electrons through a vacuum,
gas or a semiconductors.

Atomic Structures:
 Energy Band: The range of energies possessed by an electron in a solid.

Fig. 1.1.1 Atomic Structure

There are two energy bands in solids:

1. Valence Band (VB): The range of energies possessed by valence electrons.
2. Conduction Band (CB): The range of energies possessed by free electrons.

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 Fig. 1.1.2 Atomic Structure of (a) Si (b) Ge (c) Ga (d) As

Classification of Solids:
1. Insulator: Insulators are those materials which are bad conductors of electricity. VB is
completely filled while CB is empty. (Eg > 5eV) e.g. C, wood, plastic, diamond etc.
2. Metal (Conductor): Conductors are those materials which easily allow the passage of electric
current through them. VB and CB are completely filled. (Eg = 0eV) e.g. Cu, Al, Au etc.
3. Semiconductor: The materials which have conductivity property b/w insulator & conductor are
known as semiconductors. VB is almost filled while CB is almost empty. e.g. Si, Ge, GaAs etc.

The energy diagrams for the three types of solids are:

Fig. 1.2 Energy Diagram of Insulator, Semiconductor and Conductor

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 Covalent Bonding of Si & GaAs:

(a) (b)
Fig. 1.3 Covalent Bonding of (a) Si and (b) GaAs

Comparison of Si, Ge and GaAs semiconductors:

Table 1.1 Comparison of Si, Ge and GaAs semiconductors

Electron Mobility
Semiconductor Intrinsic Carriers Band Gap
(μn in cm2/V-s)
(ni in per cubic centimeter) (Eg in eV)

Si 1.5 x 1010 1.1 1500

Ge 2.5 x 1013 0.67 3900

GaAs 1.7 x 106 1.43 8500

Note: With respect to Resistance (R)……

 Insulator materials have Negative Temperature Coefficient (means R so; I ).
 Semiconductor materials have Negative Temperature Coefficient (means R so; I ).
 Conductor materials have Positive Temperature Coefficient (means R so; I ).
Q. Why Si is preferred over Ge?
 Si is preferred over Ge due to the following reason:
i) High current rating
ii) High temperature stability
iii) Small leakage current (in nA)
iv) Easily available and extractable
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Type of Semiconductors:
1. Intrinsic (pure) Semiconductor: Semiconductor which is in extremely pure form.

Fig. 1.4 Intrinsic (pure) Semiconductor

2. Extrinsic (impure) Semiconductor (P & N Type): In order to increase conductivity of
intrinsic semiconductor we have add some impurity to the intrinsic semiconductor. The
resultant is known as extrinsic type semiconductor.
 Doping: Adding of impurities (dopants) to the intrinsic semi-conductor material.
 P-type: Adding Group III dopant (or acceptor) such as B, Al, Ga, In …
 N-type: Adding Group V dopant (or donor) such as As, P, Sb, Bi …

Fig. 1.5 (a) Antimony impurity in n – type material (b) Boron impurity in p – type material

Electron versus Hole flow: If valence electron acquires sufficient energy to break its covalent
bond and fills the void created by hole, then a vacancy or hole will be created in the covalent bond
that released the electron. Now, There is a transfer of electrons to the right and holes to the left as
shown in figure 1.6.

Fig. 1.6 Electron versus hole flow

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The p-n Junction, Formation of depletion region and Barrier potential:
 The interface in-between p-type and n-type material is called a p-n junction.
 In p-side majority carriers are holes and minority carriers are electrons while in n-side majority
carriers are electrons and minority carriers are holes.
 Due to these concentration gradient difference n-side electrons starts moving towards p-side and
p-side holes start moving towards n-side.
 Due to this movement recombination near the p-n junction takes place. This whole process is
known as diffusion process.
 In p region, the free electrons diffusing from n-side recombine with the holes of the atoms. Thus
due to gain of additional negatively charged free electrons, these atoms become negative
immobile ions, just near the junction in p-region.
 In n region, the holes diffusing from p-side recombine with free electrons. Thus due to additional
positively charged holes, these atoms on n-side become positive immobile ions, just near the
junction in n-region.
 As more holes diffuse on n-side, large immobile positive charge accumulates near the junction on
n-side. This positive charge repels the positively charged holes and the diffusion of holes stop.
 Similarly large negative charge accumulates near the junction on p-side. This negative charge
repels the negatively charged electrons and the diffusion of electron stops.
 Thus there exists a wall near the junction with negative immobile charge on p-side and positively
immobile charge on n-side. There are no charge carriers in this region.
 The region is depleted off the charge carriers hence called depletion region, depletion layer or
space charge region.
 The physical distance from one side to other side of depletion region is called width of depletion
region (0.5 to 1 micron).

Fig. 1.7 Formation of depletion region

 Due to immobile positive ions on n-side and immobile negative ions on p-side, there exists an
electric field across the junction.
 This potential difference across the p-n junction is called barrier potential or height of
barrier or built-in-potential or cut-in-potential of p-n junction.

Effect of Temperature on Barrier Potential:

 We know that as temperature increase, the width of depletion region decreases.
 Thus as temperature increase, the barrier potential decreases.
 The barrier potential decreases by approximately 2.5 mV per degree Celsius rise in temperature.
 The physical distance from one side to other side of depletion region is called width of depletion
region.
 The potential difference across the p-n junction is called barrier potential or height of barrier.

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Symbol of a p-n junction Diode: The device which has two electrode i.e. anode and cathode

.
Fig. 1.8 Symbol of p-n junction diode

 No movement of charge carriers through a p-n junction at equilibrium.

 Diode ideally conducts in only one direction.

Fig. 1.9 Diode acts as Short circuit in FB and open circuit in RB

Ideal Diode Characteristics:

Fig. 1.10 (a) Conduction and (b) nonconduction states of the ideal diode as determined by theapplied bias

Biasing of p-n junction or Practical VI characteristics of p-n junction diode:

 Forward Bias (FB): When dc voltage positive terminal connected to the p region and
negative to the n region.
if VD (VBIAS ) ≥ VB
then width of depletion region decreases.

Fig 1.11 Forward Biased p-n junction when VBIAS (VF ) ≥ VB

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  Knee Voltage: The minimum forward voltage after which diode current increase
exponential. Its value for Si is 0.7 V and for Ge is 0.3 V.
 Reverse Bias (RB): dc voltage negative terminal connected to the p region and positive to
the n region. Here majority-carrier current ceases. Here width of depletion region increases.

Fig 1.12 Reverse Biased p-n junction

 However, there is still a very small current produced by minority carriers known as Reverse
Saturation Current.
 Reverse Saturation Current (Is) depends on temperature.
 The reverse saturation current double for every 100C rise in temp.

Practical V-I (volt-ampere) characteristics of p-n junction diode:

Fig 1.13 V-I characteristics of practical p-n junction diode

Diode current equation (Shockley Equation):

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Comparison of V-I characteristics of Ge, Si & GaAs semiconductor PN-Junction
diode:

Fig 1.14 Comparison of V-I characteristics of Ge, Si & GaAs semiconductor PN-Junction diode
Diode Resistance: Resistance offered by diode is known as diode resistance.
There are two type of resistance:
1. DC (Static) Resistance
2. AC (Dynamic) Resistance

DC (Static) Resistance:

Fig 1.15 Determining the dc resistance of a diode at a particular operation point

AC (Dynamic) Resistance:

Fig 1.16 Determining the ac resistance of a diode at a particular operation point

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Dynamic resistance (rd) from diode current equation: Shockley diode current equation is
given by;

(1)
OR, (2)
Now differentiating equation no. 1 wrt VD; we get

Now using equation no. 2 we get;

Since;

Therefore;
Where 𝜂 = 1 (for Ge) and 𝜂 = 2 (for Si) and

Example: 1 Calculate the dynamic forward and reverse resistance of pn junction diode when the
applied voltage is 0.5 V at temperature of 120 degree Celsius and reverse saturation current of 5 µA.
Solution: Given, VD = 0.5 V, Io = 5 µA, T = 120ºC

Since,

Here we consider pn junction diode is made up of Si material (𝜂=2).

Now for FB VD = + 0.5 V,

Hence dynamic forward resistance;

Now for RB VD = - 0.5 V,

Hence dynamic reverse resistance;

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 Example: 2 Determine the dc resistance levels for the diode shown in fig.

Solution:

Example: 3 For the characteristics of fig.

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Solution:

Diode Capacitance: Capacitance offered by diode is diode capacitance.

There are two type of capacitance:
1. In forward biased, Storage Capacitance or Diffusion Capacitance (CD) exists. It is given
as:

It is called diffusion capacitance to account for the time delay in moving charges across the
junction by diffusion process.
If in a FB junction the applied voltage is suddenly reversed, then forward current I F ceases at
once but a lot of majority charge carriers are left in the depletion region. This charge represents a
stored charge in the reverse bias condition and should be removed from space charge region. This
removal of charge takes a finite time. This effect is similar to discharge of a capacitor. Therefore,
the amount of stored charge represents the magnitude of diffusion capacitance.
From formulae it is clear that when FB voltage increases depletion width is decreases hence
diode current increase, so diffusion capacitance is also increases.

2. In reverse biased, Depletion or Transition Capacitance (CT) exists. It is given as:

As the RB voltage increases depletion width is increases, so transition capacitance is decreases.

Fig 1.17 Reverse Biased p-n junction

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 As the charges particles move away from the junction there exist a change in charge wrt the
applied reverse voltage. So change in charge (dq) wrt to change in voltage (dv) is nothing but
capacitive effect. Such capacitance which comes in the picture under RB condition is known as
transition capacitance, space charge capacitance, barrier capacitance or depletion layer
capacitance.

Characteristics of Diode capacitance:

Fig. 1.18 Diffusion and Transition capacitance versus applied bias for a Si diode

Diode Equivalent Circuit:

a. Piecewise linear equivalent circuits:

Fig. 1.19 (a) Piecewise linear equivalent circuits

b. Simplified equivalent circuits:

Fig. 1.19 (b) Simplified equivalent circuits

c. Ideal equivalent circuits:

Fig. 1.19 (c) Ideal equivalent circuits

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Example: 4 For the series diode configuration shown in fig. determine VD, VR and ID.

Solution: Since the diode is FB. So,

Example: 5 Determine Vo and ID for the series circuit shown in fig.

Solution: First drawing equivalent ckt;

Example: 6 Determine ID, VD2 and Vofor the circuit shown in fig.

Solution: First drawing equivalent ckt and then analysing ckt;

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Example: 7 Determine Vo, I1, ID1 and ID2 for the parallel diode configuration shown in fig.

Solution: Si diode are ON if the applied voltage is greate than 0.7 V.

Therefor Vo = 0.7 V

Example: 8 Determine the current ID for the network.

Solution: Si diode is conducting and GaAs diode is non-conducting because barrier or knee voltage
os Si is less than GaAs.

Therfore Current;

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Diode Applications: 1) Rectifiers 2) Voltage Multipliers 3) Clippers 4) Clampers

Rectifier Circuits: Rectifiers convert AC voltage to pulsating DC voltage (DC + AC ripples).

Type of Rectifiers:
1. Half-Wave Rectifier: This conducts only during positive half cycles of input ac supply.

Fig. 1.20 Half Wave Rectifier (HWR)

Working of Half-Wave Rectifier:

Fig. 1.21 Working of Half Wave Rectifier (HWR)

Peak Inverse Voltage (PIV) for HWR: PIV is the maximum voltage appears across the
diode during non-conducting or off state. If PIV of a diode in circuits is more than specified value
then diode may be burn out.

Fig. 1.22 Determining the required PIV Rating for the HWR

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Example: 9 (a) Sketch the output Vo and determine the dc level for the network.
(b) Repeat part (a) if the ideal diode is replaced by silicon (Si) diode.

Solution: (a)In this network diode will conduct during negative half cycle only and Vo will appear
as shown in the same figure (because we consider ideal diode in rectifier if diode material is not
specified i.e. either Si or Ge)). For the full period, the dc level is

The negative sign indicates that the polarity of the output is opposite to the defined polarity.

(b) For the silicon diode, dc level is

The resulting drop in dc level is 0.22V or about 3.5% and the output waveform appearance is:

2. Full-Wave Rectifier Circuits: This conducts for both cycles of input ac supply. It has two
types.
 Center-Tapped (CT) Full-Wave Rectifier (used two diodes)
 Full-Wave Bridge Rectifier (used four diodes)

Fig. 1.23 (a) Center-Tapped (CT) Full-Wave Rectifier Fig. 1.23 (b) Full-Wave Bridge Rectifier

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Working of Full-Wave Center-Tapped (CT) Rectifier: At a time only one diode conducts
as shown in fig. 1.24. For positive half cycle only diode D1 conducts and for negative half cycle only
diode D2 conducts.

Fig. 1.24 Working of Center-Tapped Full-Wave Rectifier

Working of Full-Wave Bridge Rectifier: At a time two diode conducts as shown in fig. 1.25.
For positive half cycle diode D2 & D3 conducts and for negative half cycle diode D4 & D1 conducts.

Fig. 1.25 Working of Full-Wave Bridge Rectifier

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PIV for Full wave Center-Tapped (CT) and Full wave Bridge rectifier: PIV is the
maximum voltage appears across the diode during non-conducting or off state.

Fig. 1.26 Determining the required PIV Rating for Full wave (a) CT rectifier (b) Bridge rectifier

Using KVL in both the circuits, we get;

PIV = Vsecondary + vo - Vm + PIV = 0
PIV = Vm + Vm = 2 Vm PIV = Vm
Example: 10 Determine the output waveform for the network and calculate the output dc level and
required PIV of each diode.

Solution: For (+)ve half cycle;

Using voltage divider rule;

The effect of removing two diodes from the bridge configuration was therefore to reduce the
available dc level to the following:
Vdc = 0.635(Vm) = 0.635(5V) = 3.18V
and PIV = vo = Vm = 5V

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Ripple Factor: The output voltage or load current of a rectifier contains two components namely
dc component and ac component. The ac component present in the output (o/p) is called a ripple.
These ripples cause pulsations in the output (o/p) of rectifier. So the effectiveness of a rectifier
depends on the magnitude of ripple in the output. Smaller the ripple more effective is the rectifier.
Mathematically, Ripple Factor (r) is the ratio of; the rms value of ac component in o/p voltage to the
dc component of o/p voltage.

Where; Vr(rms) = the rms value of the ac component of the o/p voltage
Vdc = the avg or dc value of the o/p voltage
Ir(rms) = the rms value of the ac component of the o/p current
Idc = the avg or dc value of the load current
But, the rms value of the rectified o/p load current is given by the expression

Dividing both sides by Idc and rearranging the terms so that we get

(1)
Ripple Factor for a HWR: We know that for HWR;

and
Now putting these values in equation no. 1, we get r = 1.21 = 121%.

Ripple Factor for a FWR: We know that for FWR;

and

Now putting these values in equation no. 1, we get r = 0.482 = 48.2%.

Comparison between HWR and FWR:

Table 1.2 Comparisons between HWR and FWR
Full Wave
S.No. Parameters Half Wave Rectifier
Center-Tapped Rectifier Bridge Rectifier
Conducts during Conducts during both the
1 Operation Conducts during both the half cycles
positive half cycles. half cycles
2 Number of diodes 1 2 4
The average (dc)
3 Vm / π. 2Vm / π. 2Vm / π.
load voltage
4 RMS load current Im/2. Im 2. Im 2.

5 Ripple Factor 1.21 0.48 0.48

6 Efficiency 41%. 81.2%. 81.2%.

7 PIV Vm 2Vm Vm

TUF (Transformer
8 0.287 0.69 0.81
Utilization Factor)

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Voltage Multiplier Circuits: Voltage - multiplier circuit‟s produce a dc output voltage that is
some multiple of the peak ac input voltage to this circuit.

Note: Voltage multiplier circuits are the combination of CD or DC, where C is capacitance and D is
diode.
 On the basis of multiplying factor, voltage multiplier circuit can be classified as:

 Voltage Doubler Circuit

 Voltage Tripler Circuit
 Voltage Quadrupler Circuits

 Voltage doubler circuit will produce a dc output voltage that is twice the peak ac input
voltage to the multiplier circuit. There are two types of voltage doubler circuits:

 Half Wave Voltage Doubler

 Full Wave Voltage Doubler

Working of Half wave Voltage Doubler Circuit: During the positive half cycle of the ac
input signal, the diode D1 conducts (and D2 is cut-off) and charge the capacitor C1 upto the peak
rectified voltage i.e. Vm.

During the negative half cycle of the ac input signal, the diode D2 conducts (and D1 is cut-off) and
charge the capacitor C2. Now the voltage across capacitor C1 is in the series with the input voltage.
Therefore the total voltage present to the capacitor C2 is equal to 2Vm.

Fig. 1.27 Working of Half wave Voltage Doubler Circuit

Working of Full wave Voltage Doubler Circuit: Figure shows another voltage doubler
circuit as full wave voltage doubler.

Fig. 1.28 Full wave Voltage Doubler Circuit

During the positive half cycle of the ac input signal, the diode D 1 conducts (D2 is cut-off) and charge
the capacitor C1 upto the peak rectified voltage i.e. Vm with polarity as shown in figure 1.29 (a).
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During the negative half cycle of the ac input signal, the diode D 2 conducts (D1 is cut-off) and charge
the capacitor C2 to Vm with polarity as shown in figure 1.29 (b). If there is no load connected across
the output then the output voltage is equal to 2Vm.

Fig. 1.29 Working of Full wave Voltage Doubler Circuit (a) (+) ve half cycle and (b) (-) ve half cycle

Working of Full wave Voltage Tripler and Quadrupler Circuits: During the first
positive half cycle of the ac input signal, the capacitor C1 is charged through diode D1 to a peak
voltage Vm. During the negative half cycle of the ac input signal, the capacitor C 2 is charged through
diode D2 to a peak voltage 2Vm produced by the sum of voltage across capacitor C1 and input signal.

Now, during the second positive half cycle, the diode D3 will conduct and the voltage across
capacitor C2 will charge the capacitor C3 to the 2Vm peak voltage. During negative half cycle, diode
D4 will conduct so that the capacitor C3 will charge the capacitor C4 to the same 2Vm peak voltage.
From figure 1.45, it is clear that the voltage across C2 is 2Vm, across C1 and C3, the voltage is 3Vm
and across C2 and C4, the voltage is 4V m. It means that a voltage quadrupler can provide three
different voltages i.e. 2Vm, 3Vm, 4Vm.

Fig. 1.30 Working of Full wave Voltage Tripler and Quadrupler Circuit

Limitations of voltage multipliers:

 With increasing the multiplying factor „n‟ the number of components required goes on increasing.
 Voltage regulation is poor.
 Ripple contents in the output increases with increase in load current.
 These are suitable only for low current applications.
Applications:
 They are suitable for high voltage low current applications.
 In CRT (where high voltage is required).

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Breakdown Mechanism: The breakdown voltage (1.8 V to 200 V) is controlled by the doping
level. There are two types of breakdown mechanism in the reverse-bias.
1. Zener Breakdown
 Breakdown occurs due to intense electric field (108 V/m).
 Occurs at lower voltage (<6V).
 Zener occurs when doping is high.
 Current in breakdown increases sharply.
 Vz has Negative temperature coefficient.
2. Avalanche Breakdown
 Breakdown is due to avalanche multiplication.
 Occurs at higher voltage (>6V).
 Avalanche occurs when doping is low.
 Current in breakdown increases gradually.
 Vz has Positive temperature coefficient.

Fig. 1.31 (a) Symbol of Zener diode Fig. 1.31(b) Characteristics of Zener diode

Comparison between Zener and Avalanche diodes:

Table 1.3 Comparison between Zener and Avalanche diodes
S.No. Zener Diode Avalanche Diode
1 Breakdown occurs due to intense Breakdown is due to avalanche multiplication.
electric field (108 V/m).
2 Occurs at lower voltage (<6V). Occurs at higher voltage (>6V).
3 Zener occurs when doping is high. Avalanche occurs when doping is low.
4 Current in breakdown increases Current in breakdown increases gradually.
sharply.
5 Vz has Negative temperature coefficient. Vz has Positive temperature coefficient.

Zener-Diode Voltage-Regulator Circuits:

 Zener produces constant output voltage while operating from a variable supply voltage. Such
circuits are called voltage regulator.
 The Zener diode has a breakdown voltage equal to the desired output voltage.
 The resistor limits the diode current to a safe value so that Zener diode does not overheat.

Fig. 1.32 (a) Zener-Diode Voltage-Regulator Circuits diode Fig. 1.32(b) Characteristics of Zener diode

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Using KVL; Vs = I.R + Vz
on solving we get

Now, using KCL at node (A); I = Iz + IL

Where; Iz = current across zener diode
IL = current across load resistance
Case I: Regulation with Varying Input Voltage (V in):

Case II: Regulation with Varying Load (RL):

Example: 11 (a) For the Zener diode network, determine VL, VR, IZ, and PZ
(b) Repeat part (a) with RL= 3 kΩ

Solution: (a) Note: First we check wheather Zener is ON or OFF. If V ≥ VZ, then Zener is ON
otherwise it is OFF. Now;

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Here; V < VZ i.e. 8.73 V < 10 V, So Zener is OFF means not conducting. Now we can redraw the
given network as:

Hence; VL = V = 8.73 V, VR = Vi - VL = 16 – 8.73 = 7.27 V, IZ = 0 A, PZ = VZIZ = 10 (0) = 0 W

(b) Now; RL= 3 kΩ

Again here we check whether Zener is ON or OFF. For this we calculate V across Zener i.e.

Here; V > VZ i.e. 12 V > 10 V, So Zener is ON means conducting.

Hence; VL = VZ = 10 V, VR = Vi - VL = 16 – 10 = 6 V,

Example: 12 (a) For the network, determine the range of RL and ILthat will result in VRL being
maintained at 10 V.
(b) Determine the maximum wattage rating of the diode.

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Solution: (a) In these type of Zener numericals Zener is always in ON condition.

(b) PZMAX = VZ * IZM = (10 V) (32mA) = 320 mW

Example: 13 Determine the range of values of Vi that will maintain the Zener diode in the ON state.

Solution: In these type of Zener numericals Zener is always in ON condition.

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Clipper Circuits: Clipper or Limiter circuit is used to cut off or eliminate an unwanted section of
a waveform. (In clipper numerical, if diode material is not specified then consider it as Si diode
always).
Note: Clipper circuits are the combination of DR or RD or RDR; where D and R stand for DIODE
and RESISTANCE respectively.
 There are two types of clippers:
 Series Clipper: In Series clipper diode is connected in series with load resistance.
 Shunt Clipper: In Shunt or parallel clipper diode is connected in parallel with load
resistance.
 If the diode is biased then clipper circuits are known as biased clipper otherwise it is known
as unbiased clipper circuits.
Series Clipper:

Fig. 1.33 Series Clipper

Shunt Clipper:

Fig. 1.34 Shunt Clipper

Simple Series Clippers (Ideal Diodes):

Fig. 1.35 Simple Series Clipper

Simple Parallel (Shunt) Clippers (Ideal Diode):

Fig. 1.37 Simple Parallel (Shunt) Clipper

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Biased Series Clippers (Ideal Diodes):

Fig. 1.37 Biased Series Clipper

Biased Parallel (Shunt) Clippers (Ideal Diode):

Fig. 1.38 Biased Parallel (Shunt) Clipper

Combination Clipper Circuits: Combination clipper circuit is the combination of two clipper
circuits. In this clipper circuits diode D1 conducts during (+)ve half cycle, only when applied voltage
is greater than or equal to V1. When D1 is forward biased then it maintains voltage V1 across it which
appears at the output as shown in figure. Similarly, diode D 2 conducts during (-)ve half cycle, only
when applied voltage is greater than or equal to V2. When D2 is forward biased then it maintains
voltage V2 across it which appears at the output as shown in figure.

Fig. 1.39 Combination Clipper

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Solved Example: 14 Sketch the output.

Example: 15 Determine the output waveform for the network.

Solution: The diode will be in the ON state for positive half cycle. Here diode is forward biased
by V = 5 V. So for vi = + 20 V output waveform should be start from +5 V to +25 V and
for vi = - 20 V output waveform should be start from +5 V to 0 V.

Example: 16 Repeat examples 15 for the square wave input.

Solution: For vi = 20V (0 to T/2) the diode is in short circuit (ON) state and vo = 20 + 5 = 25 V.
For vi = - 10V the diode is in open circuit (OFF) state and vo = iRR = (0) R = 0 V.

Note: In example no.15 that the clipper not only clipped off 5 V from the total swing but raised the
dc level of the signal by 5 V.

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Example: 17 Determine vo for the network (consider diode is ideal).

Solution: The polarity of the dc supply and direction of the diode strongly suggest that the diode
will be in the ON state for a good portion of the negative region of the input signal.
For this region (For (-)ve cycle) the network will appear as shown in figure, where the
defined terminals for vo = V = 4 V.

For (+)ve cycle, when the input voltage greater than 4 V the diode will be in OFF state. But for any
input voltage less than 4 V will result in a short circuited diode (ON state).

Example: 18 Repeat example 17 for VT = 0.7 V (means for Si diode).

Solution: Here; when the input voltage greater than 3.3 V the diode will be in OFF state. But for
any input voltage less than 3.3 V will result in a short circuited diode (ON state).

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Clamper Circuits: Clamper circuits are used to add a dc level to an ac input waveform.
Note: Clamper circuits are the combination of CDR; where C, D and R stand for
CAPACITANCE, DIODE and RESISTANCE respectively. (In clamper numerical, if diode
material is not specified then consider it as ideal diode always).
 On the basis of dc shift (positive or negative) clampers circuits of two types:
1. Positive Clamper Circuit (DIODE in upward direction)
2. Negative Clamper Circuit (DIODE in downward direction)
 If the diode is biased then clamper circuits are known as biased clamper otherwise it is known
as simple clamper circuits (Unbiased).

Fig. 1.40 Positive Clamper Circuit

Fig. 1.41 Negative Clamper Circuit

Steps for analysis of a clamper circuit:
1. First charge the capacitor by choosing appropriate input cycle so that diode is conducting.
2. Now calculate the output voltage as vo = vi + Vc (for positive clamper)
and vo = vi - Vc (for negative clamper)
3. Check output swing is equal to input swing i.e. 2Vm OR 2V.

Working of Negative Clamper Circuit:

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Biased Clamper Circuits: When a dc supply is used in the clamper circuits then they are
known as:
1. Biased Positive Clamper 2. Biased Negative Clamper

Fig.1.43 Biased Positive Clamper

Fig.1.44 Biased Negative Clamper

Different Types of Clamping Circuits:

Fig.1.45 Clamping circuits with ideal diodes

Example: 19 Determine vo for the network and sketch the waveform.

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Solution: This is biased positive clamper circuit. For capacitor charging we have to take negative half
cycle of period t 1 to t2.
Also we can observe that diode is forward biased by 5 V battery so total voltage across the capacitor is
appear as VC = 20 + 5 = 25 V.
Since frequency is 1000 Hz the time period will be 1 mS.
Now the output voltage for positive clamper is given as vo = vi + VC
For positive cycle vo = 10 + 25 = 35 V
For negative cycle vo = - 20 + 25 = 5 V
Hence final waveform is as follows:

Example: 20 Repeat examples 19 if diode is not ideal.

Solution: This is biased positive clamper circuit. But in this numerical diode is not ideal. So we
consider it as practical (Si) diode with barrier potential 0.7 V.
Now, for capacitor charging we have to take negative half cycle of period t 1 to t2.
Also we can observe that diode is forward biased by 5 V – 0.7V = 4.3 V battery so total voltage across
the capacitor is appear as VC = 20 + 5 – 0.7 = 24.3 V.
Since frequency is 1000 Hz the time period will be 1 mS.
Now the output voltage for positive clamper is given as vo = vi + VC
For positive cycle vo = 10 + 24.3 = 34.3 V
For negative cycle vo = - 20 + 24.3 = 4.3 V
Hence final waveform is as follows:

Special Purpose Diodes: There are four types of special purpose diodes in syllabus.
1. Light Emitting Diode (LED)
2. Liquid - Crystal Displays (LCD)
3. Varactor Diode or Tuning Diode or Varicap or Voltage Variable Capacitor (VVC)
4. Tunnel Diodes
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Light Emitting Diode (LED):
 An LED emits light when it is forward biased, which can be in the infrared or visible
spectrum.
 The forward bias voltage is usually in the range of 2 V to 3 V.

Fig. 1.46 Working of LED and symbol of LED

Materials of Light Emitting Diode (LED):

Table 1.4 Materials of Light Emitting Diode (LED)

S. No. Material Symbol Colour

GaAs Infrared (invisible)

1 Gallium Arsenide

GaP Red or Green

2 Gallium Phosphide

GaAsP Red or Yellow

3 Gallium Arsenide Phosphide

Advantages:
1. Small in size.
2. Fast operating devices (ON/OFF time is < 1 μs).
3. Light in weight.
4. Long life.
5. Cheap and readily available.
Disadvantages:
1. LED characteristics are affected by temperature.
2. Luminous efficiency of LEDs is low (about 1.5 lumen/watt).
3. Need large power for the operation compared to normal p-n junction diode.
Applications:
1. Seven Segment Display
2. In the optical devices.
3. As ON-OFF indicator.
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Liquid Crystal Display (LCD): The Liquid Crystals are one of the most fascinating material
systems in nature having properties of liquids as well as of solid crystals.
 The different arrangement of these liquid Crystals materials are:
1. Smectic
2. Nematic
3. Cholestrics

Fig. 1.47 Liquid Crystal Displays (LCD) (a) Nematic with no applied bias (b) Nematic with applied bias

Advantages:
1. Less power consumption.
2. Low cost.
3. Uniform brightness with good contrast.
4. Low operating voltage and current.
Disadvantages:
1. Poor reliability.
2. Limited temperature range.
3. Poor visibility in low ambient temperature.
4. Slow speed.
5. Requires an AC drive.
Applications:
1. 7-segment displays.
2. Calculators, pagers and wrist-watch.
3. Flat panel display.
4. Alphanumeric display.

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Difference between LEDs and LCDs:
Table 1.5 Differences between LEDs and LCDs

S. No. Parameter LED LCD

1 Power 10 to 140 μW 1 to 100 μW

2 Voltages 5V 3 to 18 V

3 Temperature Range (0C) -55 to 125 0 to 80

4 Switching Speeds 1 μs 100 to 300 ms

5 Life in hours 100,000 + 10,000

Red, Orange, Yellow,
6 Colors Depends on illumination
Green
7 Brightness Good to excellent Not applicable

8 Ruggedness Excellent Good

9 Ease of mounting Excellent Poor

1. 7-segment Displays
2. Calculators, pagers and
1. 7-segment displays
wrist-watch
10 Applications 2. Indicator
3. Flat panel display
3. Matrix displays
4. Alphanumeric display

Varactor Diode or Tuning or Varicap or Voltage Variable Capacitor (VVC):

 Varactor Diode or Tuning or Varicap or VVC means the diode which acts as a Variable
Capacitor under changing reverse bias.

Fig 1.48 Reverse Biased p-n junction

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  The mode of operation depends on the capacitance that exists when p-n junction is reverse

biased and is given as:

 In RB as V Wd CT as shown in graph:

Fig. 1.49 Characteristics of Varactor or Tunning or Varicap diode

Fig. 1.50 Symbol of Varactor or Tunning or Varicap diode

Applications:
1. Tuned circuits.
2. FM modulators.
3. Automatic frequency control devices.
4. Adjustable band pass filters.
5. Television receivers.

Tunnel Diode: Highly doped p-n junction diodes in FB condition acts as Tunnel Diode.
 A tunnel diode or Esaki diode is a type of semiconductor that is capable of very fast
operation.
 It was invented in August 1957 by Leo Esaki, Yuriko Kurose and Takashi Suzuki when they
were working at Tokyo Tsushin Kogyo, now known as Sony.

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  The diodes in which the concentration of impurity atoms is upto 1 part in 10 3 wrt normal p-n
junction diode in which impurity concentration is 1 part in 10 8.
 The tunnel diode exhibits negative resistance.
 It will actually conduct well with low forward bias.
 Further increases in bias it reaches the negative resistance range where current will actually
go down.
 This is achieved by heavily-doped p and n materials that create a very thin depletion region
which permits electrons to “tunnel” through the barrier region.

Fig. 1.51 Symbol of Tunnel diode

Fig. 1.52 Characteristics of Tunnel diode

Applications:
1. High frequency oscillator.
2. High speed switching networks.
3. In pulse and digital circuits.
4. In timing and computer logic circuitry.
5. In pulse generators and amplifiers.

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 FAQs
1. Why Si is preferred over Ge? (2015-16)
2. Write short notes on classification of solids. (2008-09)
3. Explain the Intrinsic and Extrinsic semiconductors. (2014-15)
4. Explain the formation of Depletion layer along with barrier potential and effect of temperature on
barrier potential. (2010-11, 2012-13)
5. Draw the symbol of p-n junction diode. (2010-11, 2012-13)
6. Explain knee voltage & its value for Si and Ge diodes. (2006-07)
7. Draw the V-I characteristics for Ideal and Practical diode. (2009-10)
8. Write the diode current equation and explain each term.(2005-06, 2010-11)
9. Explain Diode capacitance in reference to diode. (2010-11)
10. Define Rectifier. Draw the circuit diagram of Half Wave Rectifier and explain its operation with
output waveforms. (2010-11, 2012-13)
11. Draw the circuit diagram of Full Wave Rectifier (Centre tapped) and explain its operation with
output waveforms. (2001-02, 2004-05, 2009-10, 2011-12, 2012-13, 2015-16)
12. Draw the circuit diagram of Full Wave Rectifier (Bridge) and explain its operation with output
waveforms. (2001-02, 2004-05, 2009-10, 2011-12, 2015-16)
13. Explain PIV for FWR (Centre tapped & Bridge). (2011-12)
14. Draw the symbol of Zener diode. (2009-10)
15. Compare Zener and Avalanche breakdowns. (2006-07)
16. Compare Zener and p-n junction diode. (2007-08)
17. How Zener works as a voltage regulator or shunt regulator. (2015-16)
18. Explain the clipper circuits and its various types. ( 2002-03, 2012-13, 2015-16)
19. Explain the classification and operation with input & output waveform of clamper circuit. (2006-
07, 2011-12, 2014-15, 2015-16)
20. Voltage Multiplier Circuits? Explain any one. (2006-07, 2015-16)
21. Explain the Light Emitting Diodes (LED) with neat diagram. (2013-14)
22. Compare LED and LCD. (2014-15)
23. Compare LED and p-n junction diode. (2009-10)
24. Explain the Tuning (Varactor diode or VVC) diode with its characteristics.(2013-14)
25. Explain Tunnel diode with its characteristics. (2012-13)

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 SUGGESTED ANSWER OF FAQs

1. Why Si is preferred over Ge? (2015-16) 2 MARKS

Ans. Si is preferred over Ge due to the following reason:
i) High current rating
ii) High temperature stability
iii) Leakage current is small (in nA)
iv) Easily available and extractable
2. Explain the Intrinsic and Extrinsic semiconductors. (2014-15) 2 MARKS
Ans. Intrinsic (pure) Semiconductor: Semiconductor which is in extremely pure form.
e.g. Si, Ge etc.

Extrinsic (impure) Semiconductor (P & N Type): In order to increase conductivity of intrinsic

semiconductor we have add some impurity to the intrinsic semiconductor. The resultant is known as
extrinsic type semiconductor.

 Doping: Adding of impurities (dopants) to the intrinsic semi-conductor material.

 P-type: Adding Group III dopant (or acceptor) such as B, Al, Ga, In …
 N-type: Adding Group V dopant (or donor) such as As, P, Sb, Bi …

Fig. (a) Antimony impurity in n – type material (b) Boron impurity in p – type material

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3. Explain the formation of Depletion layer along with barrier potential and effect of temperature
on barrier potential. (2010-11, 2012-13) 5 MARKS
Ans. The interface in-between p-type and n-type material is called a p-n junction.
 In p-side majority carriers are holes and minority carriers are electrons while in n-side
majority carriers are electrons and minority carriers are holes.
 Due to these concentration gradient difference n-side electrons starts moving towards p-
side and p-side holes start moving towards n-side.
 Due to this movement recombination near the p-n junction takes place. This whole
process is known as diffusion process.

Fig. (a) p-n - junction

 In p region, the free electrons diffusing from n-side recombine with the holes of the atoms.
Thus due to gain of additional negatively charged free electrons, these atoms become
negative immobile ions, just near the junction in p-region.
 In n region, the holes diffusing from p-side recombine with free electrons. Thus due to
additional positively charged holes, these atoms on n-side become positive immobile ions,
just near the junction in n-region.
 As more holes diffuse on n-side, large immobile positive charge accumulates near the
junction on n-side. This positive charge repels the positively charged holes and the diffusion
of holes stop.
 Similarly large negative charge accumulates near the junction on p-side. This negative charge
repels the negatively charged electrons and the diffusion of electron stops.
 Thus there exists a wall near the junction with negative immobile charge on p-side and
positively immobile charge on n-side. There are no charge carriers in this region.
 The region is depleted off the charge carriers hence called depletion region, depletion layer or
space charge region.

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  The physical distance from one side to other side of depletion region is called width of
depletion region (0.5 to 1 micron).

Fig. (b) Formation of depletion region

 Due to immobile positive ions on n-side and immobile negative ions on p-side, there exists an
electric field across the junction.
 This potential difference across the p-n junction is called barrier potential or height of barrier
or built-in-potential or cut-in-potential of p-n junction.

Effect of Temperature on Barrier Potential:

 We know that as temperature increase, the width of depletion region decreases.
 Thus as temperature increase, the barrier potential decreases.
 The barrier potential decreases by approximately 2.5 mV per degree Celsius rise in temperature.

4. Draw the symbol of p-n junction diode. (2010-11, 2012-13) 2 MARKS

Ans. Symbol of a p-n junction Diode: The device which has two electrode i.e. anode and
cathode.

Fig. Symbol of p-n junction diode

5. Explain knee voltage & its value for Si and Ge diodes. (2006-07) 2 MARKS
Ans. Knee Voltage: The minimum forward voltage after which diode current increase exponential.
Its value for Si is 0.7 V and for Ge is 0.3 V.

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6. Write the diode current equation and explain each term.(2005-06, 2010-11) 2 MARKS
Ans. Diode current equation (Shockley Equation):

7. Draw the V-I characteristics for Ideal and Practical diode. (2009-10) 5 MARKS
Ans. Ideal V-I characteristics:

Practical V-I characteristics:

Fig V-I characteristics of practical p-n junction diode

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8. Explain Diode capacitance in reference to diode. (2010-11) 5 MARKS

Ans. Diode Capacitance: Capacitance offered by diode is diode capacitance.

There are two type of capacitance:
1. In forward biased, Storage Capacitance or Diffusion Capacitance (CD) exists. It is given
as:

2. In reverse biased, Depletion or Transition Capacitance (CT) exists. It is given as:

Characteristics of Diode capacitance:

9. Define Rectifier. (2010-11, 2012-13) 2 MARKS

Ans. Rectifier Circuits: Rectifiers convert AC voltage to pulsating DC voltage.

Type of Rectifiers:
1. Half-Wave Rectifier: This conducts only during positive half cycles of input ac supply.

2. Full-Wave Rectifier Circuits: This conducts for both cycles of input ac supply. It has
two types.
 Center-Tapped (CT) Full-Wave Rectifier
 Full-Wave Bridge Rectifier

10. Draw the circuit diagram of Half Wave Rectifier and explain its operation with output
waveforms. (2010-11, 2012-13) 5 MARKS

Ans. Half-Wave Rectifier: This conducts only during positive half cycles of input ac supply.

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 Fig. Half Wave Rectifier (HWR)

Working of Half-Wave Rectifier:

Fig. Working of Half Wave Rectifier (HWR)

11. Draw the circuit diagram of Full Wave Rectifier (Centre tapped) and explain its operation with
output waveforms. (2001-02, 2004-05, 2009-10, 2011-12, 2012-13, 2015-16) 5 MARKS
Ans. Working of Full-Wave Center-Tapped (CT) Rectifier: This conducts for both
cycles of input ac supply. It has two types.

Fig. Working of Center-Tapped Full-Wave Rectifier

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 12. Draw the circuit diagram of Full Wave Rectifier (Bridge) and explain its operation with output
waveforms. (2001-02, 2004-05, 2009-10, 2011-12, 2015-16) 5 MARKS
Ans. Working of Full-Wave Bridge Rectifier:

Fig. Working of Full-Wave Bridge Rectifier

13. Explain PIV and Ripple factor for FWR (Centre tapped & Bridge). (2011-12) 4 MARKS
Ans. PIV for Full wave Center-Tapped (CT) and Full wave Bridge rectifier:

Fig. Determining the required PIV Rating for Full wave (a) CT rectifier (b) Bridge rectifier

Using KVL;
PIV = Vsecondary + VR - Vm + PIV = 0
PIV = Vm + Vm = 2 Vm PIV = Vm
14. Draw the symbol of Zener diode. (2009-10) 2 MARKS
Ans.

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15. Compare Zener and Avalanche breakdowns. (2006-07) 5 MARKS
Ans. Comparison between Zener and Avalanche breakdown:

S.No. Zener breakdown Avalanche breakdown

Breakdown occurs due to intense Breakdown is due to avalanche multiplication.
1 electric field (108 V/m).
Occurs at lower voltage (<6V). Occurs at higher voltage (>6V).
2
Zener occurs when doping is high. Avalanche occurs when doping is low.
3
Current in breakdown increases Current in breakdown increases gradually.
4 sharply.
Vz has Negative temperature Vz has Positive temperature coefficient.
5 coefficient.

16. How Zener works as a voltage regulator or shunt regulator. (2015-16) 5 MARKS
Ans. Zener-Diode Voltage-Regulator Circuits:
 Zener produces constant output voltage while operating from a variable supply voltage.
Such circuits are called voltage regulator.
 The Zener diode has a breakdown voltage equal to the desired output voltage.
 The resistor limits the diode current to a safe value so that Zener diode does not overheat.

Fig. (a) Zener-Diode Voltage-Regulator Circuits diode Fig. (b) Characteristics of Zener diode

17. Explain the clipper circuits and its various types. ( 2002-03, 2012-13, 2015-16) 5 MARKS
Ans. Clipper Circuits: Clipper or Limiter circuit is used to cut off or eliminate an unwanted
section of a waveform.
Note: Clipper circuits are the combination of DR or RD or RDR; where D and R stand for DIODE
and RESISTANCE respectively.
 There are two types of clippers:
 Series Clipper: In Series clipper diode is connected in series with load.
 Shunt Clipper: In Shunt or parallel clipper diode is connected in series with load.
 If the diode is biased then clipper circuits are known as biased clipper otherwise it is known
as unbiased clipper circuits.
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18. Explain the classification and operation with input & output waveform of clamper circuit. (2006-
07, 2011-12, 2014-15, 2015-16) 5 MARKS
Ans. Clamper Circuits: Clamper circuits are used to add a dc level to an ac input waveform.
Note: Clamper circuits are the combination of CDR; where C, D and R stand for
CAPACITANCE, DIODE and RESISTANCE respectively.
 On the basis of dc shift (positive or negative) clampers circuits of two types:
3. Positive Clamper Circuit (DIODE in upward direction)
4. Negative Clamper Circuit (DIODE in downward direction)
 If the diode is biased then clamper circuits are known as biased clamper otherwise it is known
as simple clamper circuits (Unbiased).

Fig. (a) Positive Clamper Circuit

Fig. (b) Negative Clamper Circuit

19. What is Voltage Multiplier Circuits? Explain any one. (2006-07, 2015-16)
10 MARKS
Ans. Voltage Multiplier Circuits: Voltage - multiplier circuit‟s produce a dc output voltage
that is some multiple of the peak ac input voltage to this circuit.
 On the basis of multiplying factor, voltage multiplier circuit can be classified as:
 Voltage Doubler Circuit
 Voltage Tripler Circuit
 Voltage Quadrupler Circuits

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  Voltage doubler is again classified as:
 Half Wave Voltage Doubler
 Full Wave Voltage Doubler

Working of Half wave Voltage Doubler Circuit:

20. Explain the Light Emitting Diodes (LED) with neat diagram. (2013-14) 5 MARKS
Ans. An LED emits light when it is forward biased, which can be in the infrared or visible
spectrum. The forward bias voltage is usually in the range of 2 V to 3 V.

Fig. Working of LED and symbol of LED

21. Compare LED and LCD. (2014-15) 5 MARKS

Ans. Comparison between LEDs and LCDs:

S. No. Parameter LED LCD

1 Power 10 to 140 μW 1 to 100 μW

2 Voltages 5V 3 to 18 V

3 Temperature Range (0C) -55 to 125 0 to 80

4 Switching Speeds 1 μs 100 to 300 ms

5 Life in hours 100,000 + 10,000

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 Red, Orange, Yellow,
6 Colors Depends on illumination
Green

7 Brightness Good to excellent Not applicable

8 Ruggedness Excellent Good

9 Ease of mounting Excellent Poor

5. 7-segment
Displays
4. 7-segment 6. Calculators, pagers
displays and wrist-watch
10 Applications 5. Indicator 7. Flat panel display
6. Matrix displays 8. Alphanumeric
display

22. Explain the Tuning (Varactor diode or VVC) diode with its characteristics.(2013-14) 5 MARKS

Ans. Varactor Diode or Tuning or Varicap or Voltage Variable Capacitor (VVC):

Fig. Characteristics of Varactor or Tunning or Varicap diode

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23. Explain Tunnel diode with its characteristics. (2012-13) 5 MARKS
Ans. Tunnel Diode: Highly doped p-n junction diodes in FB condition acts as Tunnel Diode.
 The diodes in which the concentration of impurity atoms is upto 1 part in 10 3 w.r.t. normal p-
n junction diode in which impurity concentration is 1 part in 10 8.
 The tunnel diode exhibits negative resistance.
 It will actually conduct well with low forward bias.
 Further increases in bias it reaches the negative resistance range where current will actually
go down.
 This is achieved by heavily-doped p and n materials that create a very thin depletion region
which permits electrons to “tunnel” thru the barrier region.

Fig. (a) Symbol of Tunnel diode

Fig. (b) Characteristics of Tunnel diode

Applications:
1. High frequency oscillator.
2. High speed switching networks.
3. In pulse and digital circuits.
4. In timing and computer logic circuitry.
5. In pulse generators and amplifiers.

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