Post on 14-Oct-2020
SSathya Priyadharshini. Asst. Prof./EEE 1
MAHALAKSHMI
ENGINEERING COLLEGE
TIRUCHIRAPALLI-621213.
QUESTION BANK
DEPARTMENT: EEE SEMESTER : III
SUBJECT CODE: EE2203 SUBJECT NAME: ELECTRONIC
DEVICES &CIRCUITS
UNIT 1-PN DIODE AND ITS APPLICATIONS
PART A (2 Marks)
1. What are the advantages and limitations of LCD displays. (AUC NOV 12)
LCD displays are cheaper than LED displays. The disadvantage is that the viewing
angle is small i.e., it can be viewed only in straight angle. Images viewed from cross
angles appear blurred.
2. What is meant by dynamic resistance of diode? (AUC DEC’11)
Dynamic resistance of a diode can be defined as the ratio of change in voltage
across the diode to the change in current through the diode.
r = V / I
Where
r - Dynamic resistance of a diode
V - change in voltage across the diode
I - change in current through the diode
3. Differentiate between zener breakdown and avalanche breakdown. (AUC DEC ‘11)
Zener breakdown occurs in a reverse biased junction which gives a constant output
voltage. Avalanche breakdown does not provide a constant output voltage. This
constant voltage from a zener diode can be used as a reference voltage for many
regulators.
4. What is meant by diffusion current in a semi conductor? (AUC MAY’10)
In a non uniformly doped semi conductor charge carriers have a tedency to move
from a region of higher concentration to a region of lower concentration. The flow of
current due to this process is called diffusion current.
5. Define Knee voltage of a diode. (AUC NOV’10)
SSathya Priyadharshini. Asst. Prof./EEE 2
Knee voltage of a diode is defined as a breakover voltage above which the forward
current increases abruptly. This voltage is also called as brweakdown voltage of a
diode. It is 0.3 for Ge and 0.7 for Si.
6. What is peak inverse voltage? (AUC NOV’10)
The maximum reverse-bias potential that can be applied before entering the Zener
region is called the peak inverse voltage (referred to simply as the PIV rating) or the
peak reverse voltage (denoted by PRV rating). Peak inverse voltage is defined as the
maximum reverse voltage that a diode can be subjected to operate in a reverse
region so that the diode does not get damaged due to this reverse voltage.
7. What is LED? Which material is used for LED? (AUC MAY’09)
LED is Light Emmiting Diode. Gallium arsenide, gallium phosphide, Gallium arsenide
phosphide are the materials used for monufacturing LED which emits infra res, res,
green light respectively.
8. What is meant by doping in a semiconductor? (AUC MAY’09)
The method of adding impurities to a semiconductor to make them conduct is called
doping. It is the process of adding trivalent, pentavalent impurity to the
semiconductor which gives rise to P type and N type semiconductor.
9. Give the equation for diode current under reverse bias. (AUC MAY’08)
ID=IO [eVD/nVT-1]
10. What is diffusion capacitance? (AUC MAY’08)
Charge carriers flowing out of a depletion region, which is widened when the junction
is reverse biased, there is a large reverse current at first and which slowly decreases
to the level of reverse saturation current. The effect is like a discharging of a
capacitor called as diffusion capacitance.
11. How do the transition region width and contact potential across a PN junction vary
with the applied bias voltage? (AUC DEC’07)
The width of the PN junction is widenened for a reverse biased junction and narrows
for a forward biased junction.
12 . What is an ideal diode?
An ideal diode is one which offers zero resistance when forward biased and infinite
resistance when reverse biased.
13. Compare ideal diode as a switch.
An ideal diode when forward biased is equivalent a closed (ON) switch and when
reverse biased, it is equivalent to an open (OFF) switch.
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14. State the mathematical equation which relates voltage applied across the PN
junction diode and current flowing through it.
15. Define knee/cut-in/threshold voltage of a PN diode.
It is the forward voltage applied across the PN diode below which practically no
current flows.
16. What is the effect of junction temperature on cut-in voltage of a PN diode?
Cut-in voltage of a PN diode decreases as junction temperature increases.
17. What is the effect of junction temperature on forward current and reverse current of a
PN diode?
For the same forward voltage, the forward current of a PN diode increases and
reverse saturation current increases with increase in junction temperature.
18. Differentiate between breakdown voltage and PIV of a PN diode.
The breakdown voltage of a PN diode is the reverse voltage applied to it at which the
PN junction breaks down with sudden rise in reverse current. Whereas, the peak
inverse voltage (PIV) is the maximum reverse voltage that can be applied to the PN
junction without damage to the junction.
19. Differentiate avalanche and zener breakdowns. Avalanche Breakdown
1. Breakdown occurs due to heavily doped junction and applied strong electric field.
2. Doping level is high.
3. Breakdown occurs at lower voltage compared to avalanche breakdown Zener
Breakdown
20. Draw the V-I characteristics of an ideal diode.
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21. Differentiate between drift and diffusion currents.
Drift Current
1. It is developed due to potential gradient.
2. This phenomenon is found both in metals and semiconductors
Diffusion Current
1. It is developed to charge concentration gradient.
2. It is found only in semiconductors.
22. Draw the V-I characteristics of a practical PN diode
23. List the PN diode parameters.
1. Bulk Resistance.
2. Static Resistance/Junction Resistance (or) DC Forward Resistance
3. Dynamic Resistance (or) AC Forward Resistance
4. Reverse Resistance
5. Knee Voltage
6. Breakdown Voltage
7. Reverse Current (or) Leakage Current
24. State the PN diode ratings.
Even PN-Junction has limiting values of maximum forward current, peak
inverse voltage and maximum power rating.
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25. Define reverse recovery time.
It is maximum time taken by the device to switch from ON to OFF stage. 15. List the
PN diode switching times.
1. Recovery Time
2. Forward Recovery Time 3. Reverse Recovery Time 4. Reverse recovery time,
5. Storage and Transition Times
26. Define transition capacitance of a diode.
Transition Capacitance (CT) or Space-charge Capacitance: When a PN-junction
is reverse-biased, the depletion region acts like an insulator or as a dielectric.
The P- and N-regions on either side have low resistance and act as the plates.
Hence it is similar to a parallel-plate capacitor. This junction capacitance is called
transition or space-charge capacitance (CT).
It is given by
Where, A = Cross-sectional area of depletion region. D = Width (or) thickness of
depletion region.
Its typical value is 40 pF.
Since the thickness of depletion layer depends on the amount of reverse bias, CT
can be controlled with the help of applied bias.
This property of variable capacitance is used in varicap or varactor diode. This
capacitance is is voltage dependent and is given by
27. Define diffusion capacitance of a diode.
Diffusion or Storage Capacitance (CD): This capacitive effect is present when
the junction is forward-biased.
It is called diffusion capacitance due to the time delay in minority charges across the
junction by diffusion process. Due to this fact, this capacitance cannot be identified in
terms of a dielectric and plates. It varies directly with forward current. When a
forward-biased PN-junction is suddenly reverse biased, a reverse current flows
which is large initially, but gradually decreases to the level of saturation current, I0.
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This effect can be likened to the discharging, of a capacitor and is, therefore
called diffusion capacitance, CD. Its typical value is 0.02 F
It is given by:
28. List some applications of zener diode.
Zener diode find wide commercial and industrial applications. Some of their
common applications are:
As voltage regulators.
As peak clippers or voltage limiters.
For wave shaping.
For meter protection against damage from accidental application of
excessive voltage.
As a fixed reference voltage in a network for biasing and comparison
purposes and for calibrating voltmeters.
29. State the principle of operation of an LED.
When a free electron from the higher energy level gets recombined with the hole , it
gives out the light output. Here, in case of LEDs, the supply of higher level electrons
is provided by the battery connection.
30. State any four advantages of LED.
They are small in size.
Light in weight.
Mechanically rugged.
Low operating temperature.
Switch on time is very small.
Available in different colours.
They have longer life compared to lamp Linearity is better.
Compatible with ICs
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. Low cost.
31. State some disadvantages of LED.
Output power gets affected by the temperature radiation Quantum
efficiency is low.
Gets damaged due to over –voltage and over-current.
32. List the applications of LED.
They are used in various types of displays.
They are used as source in opto-couplers.
Used in infrared remote controls.
Used as indicator lamps.
Used as indicators in measuring devices.
33. State the principle of operation of an LCD.
Basically this type of display consists of liquid crystal molecules.
These molecules have a special property. The change their orientation when
an electrical signal is applied to them.
The display consists of two glass plates and liquid crystal molecules are
placed in between the glass plates.
When no electrical signal is applied to the liquid crystal cell, then all the
liquid crystal molecules have random orientation with respect to their axis.
The incoming light passes through the gap of molecules. So, the light also gets
twisted.
Now, when an electrical signal is applied to this structure, then all the
liquid crystal molecules gets oriented by 90° to the glass plate. In this case, this light
passes in straight way along the molecular arrangement
34. State any four advantages of LCD.
Less amount of power per digit is required.
LCDs have best contrast ratio.
No external interfacing circuitry is required.
They have low threshold voltage.
They can be driven directly.
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LCDs and MOS compatible. Small size and low cost
35. State any four application of LCD.
LCDs are generally applicable in the field of medical, domestic and
industrial electronics. Some of the applications of LCD are:
Wrist watches.
Telephones and cellular phones.
Digital panel meters.
PCO monitors.
Calculators.
For space applications.
In digital clocks.
Televisions.
Automobiles, etc.
36. Compare LEDs and LCDs.
LEDs
LCDs
1. More power is required. 2. Fastest
displays
3. More life.
4. LED is light source.
5. More temperature range. 6. Mounting is
easy
1. Less power is required. 2. Slowest
displays.
3. Less life.
4. LCD is not light source. It is a light
reflector.
5. Less temperature range 6. Mounting is
difficult.
37. Define rectifier efficiency.
It is defined as the ratio of DC power output to the applied AC power in put Rectifier
efficiency.
38. Define ripple factor of a rectifier.
The purpose of a rectifier is to convert AC into DC. But the pulsating output of
a rectifier contains a DC component and an AC component, called ripple.
The ratio of RMS value of AC components to the DC component in the rectifier
output is called ‘ripple factor’.
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The ripple factor is very important in deciding the effectiveness of a rectifier. It
indicates the purity of the DC power output. The smaller the ripple factor, the lesser
the effective AC component and hence more effective is the rectifier.
39. Define TUF of a rectifier.
Most of the rectifier circuits make use of transformer whose secondary feeds the
AC power. The transformer rating is necessary to design a power supply.
Transformer utilization factor (TF) id defined as the ratio of DC power delivered
to the load to the AC power rating of transformer secondary.
40. Give the advantages and disadvantages of HWR and FWR. Half Wave Rectifier
(HWR)
Advantages
Simple circuit Low cost.
Disadvantages.
Rectification efficiency is low (40.6%) Very high amount of ripple (γ = 1.21)
Low TUF (0.287)
Saturation of transformer core occurs.
41. What is the need for a filter in rectifier?
The output of a rectifier is pulsating and contains a steady DC component
with undesirable ripples. If such pulsating DC is given to the electronic
circuits, it produces disturbances and other interferences. Hence ripples have to
be kept far from the load.
42. What is voltage regulator? List some types.
A voltage regulator is a circuit which makes the rectifier-filter output
constant regardless of the variations in the input voltage or load.
PART B (16 Marks)
1. (i) How a PN junction diode is working? Draw and explain V-I characteristics of PN
diode with neat diagrams.
(ii) List out and explain the applications of LED and LCD (AUC NOV’12)
(i) PN junction diode
The semiconductor diode is formed by simply bringing these materials together
(constructed from the same base—Ge or Si). At the instant the two materials are
joined the electrons and holes in the region of the junction will combine, resulting in
a lack of carriers in the region near the junction. This region of uncovered positive
and negative ions is called the depletion region due to the depletion of carriers in
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this region. Since the diode is a two-terminal device, the application of a voltage
across its terminals leaves three possibilities: no bias (VD = 0 V), forward bias (VD
>0 V), and reverse bias (VD< 0 V).
p-n junction with no external bias.
No Applied Bias (VD = 0 V)
Under no-bias (no applied voltage) conditions, any minority carriers (holes) in the n-
type material that find themselves within the depletion region will pass directly into
the p-type material. The closer the minority carrier is to the junction, the greater the
attraction for the layer of negative ions and the less the opposition of the positive ions
in the depletion region of the n-type material. Assume that all the minority carriers of
the n-type material that find themselves in the depletion region due to their random
motion will pass directly into the p-type material. Similar discussion can be applied to
the minority carriers (electrons) of the p-type material. This carrier flow has been
indicated in the above figure for the minority carriers of each material. The majority
carriers (electrons) of the n-type material must overcome the attractive forces of the
layer of positive ions in the n-type material and the shield of negative ions in the p-
type material to migrate into the area beyond the depletion region of the p-type
material. However, the number of majority carriers is so large in the n-type material
that there will invariably be a small number of majority carriers with sufficient kinetic
energy to pass through the depletion region into the p-type material. Again, the same
type of discussion can be applied to the majority carriers (holes) of the p-type
material. The resulting flow due to the majority carriers is also shown in the above
figure. In the absence of an applied bias voltage, the net flow of charge in any one
direction for a semiconductor diode is zero. The symbol for a diode is shown in the
below figure with the associated n- and p-type regions. Note that the arrow is
associated with the p-type component and the bar with the n-type region. As
indicated, for VD= 0 V, the current in any direction is 0 mA.
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Diode Symbol
Reverse-Bias Condition (VD < 0 V)
If an external potential of V volts is applied across the p-n junction such that the
positive terminal is connected to the n-type material and the negative terminal is
connected to the p-type material as shown in the below figure. The number of
uncovered positive ions in the depletion region of the n-type material will increase
due to the large number of free electrons drawn to the positive potential of the
applied voltage. For similar reasons, the number of uncovered negative ions will
increase in the p-type material. The net effect, therefore, is a widening of the
depletion region. This widening of the depletion region will establish too great a
barrier for the majority carriers to overcome, effectively reducing the majority carrier
flow to zero as shown in the below figure.
Reverse-biased p-n junction.
The number of minority carriers, however, that find themselves entering the
depletion region will not change, resulting in minority-carrier flow vectors of the
same magnitude with no applied voltage the current that exists under reverse-bias
conditions is called the reverse saturation current and is represented by Io.
Forward-Bias Condition (VD > 0 V)
A forward-bias condition is established by applying the positive potential to the p-
type material and the negative potential to the n-type material as shown in the
below figure. A semiconductor diode is forward-biased when the association p-type
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and positive and n-type and negative has been established.
Forward-biased p-n junction
The application of a forward-bias potential VD will pressure electrons in the n-type
material and holes in the p-type material to recombine with the ions near the
boundary and reduce the width of the depletion region as shown in the above
figure. The resulting minority-carrier flow of electrons from the p-type material to
the n-type material (and of holes from the n-type material to the p-type material)
has not changed in magnitude (since the conduction level is controlled primarily by
the limited number of impurities in the material), but the reduction in the width of
the depletion region has resulted in a heavy majority flow across the junction. An
electron of the n-type material now sees a reduced barrier at the junction due to
the reduced depletion region and a strong attraction for the positive potential
applied to the p-type material.
Silicon semiconductor diode characteristics.
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As the applied bias increases in magnitude the depletion region will continue to
decrease in width until a flood of electrons can pass through the junction, resulting in
an exponential rise in current as shown in the forward-bias region of the
characteristics curve shown before. Note that the vertical scale of characteristic
curve is measured in mill amperes and the horizontal scale in the forward-bias region
has a maximum of 1 V. typically, therefore, the voltage across a forward-biased
diode will be less than 1 V.
(ii) Applications of LED & LCD
LED:
Used in burglar alarm systems
For solid state video displays
In image sensing circuits used for picturephone
Optical fibre communication systems
Datalink & remote controllers
For numeric displays in hand held or pocket calculators.
LCD
Field effect LEDs are used in watches & portable instruments, where source
energy is prime consideration.
Thousands of tiny LCDs are used to form the picture elements (pixels) of the
screen in B&W pocket TV receiver.
Desktop monitors, notebook computer display
Cellular phone displays, display on personal Digital Assistants(PDA).
2. In a semiconductor at room temperature (300 degree Kelvin), the intrinsic carrier
concentration and resistivity are 1.5*106/cm3 and 2*103 Ωm respectively. It is
converted to an extrinsic semiconductor with a doping concentration of 1020/m3. For
the extrinsic semiconductor calculate the
(i) Minority carrier concentration
(ii) Resistivity
(iii) Shift in Fermi level due to doping
(iv) Minority carrier concentration when its temperature is increased to a value at
which the intrinsic carrier carrier concentration ‘n i’ doubles. Assume the
mobility of majority and minority carriers to be the same and kT= 20meV at
room temperature. (AUC NOV’12)
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3. Draw the circuit diagram and explain the operation of full wave rectifier using center
tap transformer and using bridge rectifier without center tap transformer. Obtain the
expression for peak inverse voltage. (AUC DEC’11)
4. (i) With neat diagram explain the construction and working of LED.
(ii) Explain the working of LCD seven segment displays using square wave supply.
(AUC DEC’11)
5. With a neat diagram explain the working of a PN junction diode in forward bias and
reverse bias and show the effect of temperature on its V-I characteristics.
(AUC MAY’10)
Answer is same as Q1 in partB for working of diode in forward & reverse bias. The
temperature effect is alone explained here.
Temperature Effects of pn junction diode:
Temperature can have a marked effect on the characteristics of a silicon
semiconductor diode as shown in the below figure. It has been found experimentally
that the reverse saturation current Io will just about double in magnitude for every
10°C increase in temperature.
Variation in diode characteristics with temperature change.
It is not uncommon for a germanium diode with an Io in the order of 1 or 2 A at 25°C
to have a leakage current of 100 A _ 0.1 mA at a temperature of 100°C. Typical
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values of Io for silicon are much lower than that of germanium for similar power and
current levels. The result is that even at high temperatures the levels of Io for silicon
diodes do not reach the same high levels obtained for germanium—a very important
reason that silicon devices enjoy a significantly higher level of development and
utilization in design. Fundamentally, the open-circuit equivalent in the reverse bias
region is better realized at any temperature with silicon than with germanium. The
increasing levels of Io with temperature account for the lower levels of threshold
voltage. Simply increase the level of Io in and not rise in diode current. Of course, the
level of TK also will be increase, but the increasing level of Io will overpower the
smaller percent change in TK. As the temperature increases the forward
characteristics are actually becoming more ideal.
6. Explain V-I characteristics of Zener diode. (AUC MAY’10)
Zener Region:
There is a point where the application of too negative a voltage will result in a sharp
change in the characteristics, as shown in the below figure. The current increases at
a very rapid rate in a direction opposite to that of the positive voltage region. The
reverse-bias potential that results in this dramatic change in characteristics is called
the Zener potential and is given the symbol VZ. As the voltage across the diode
increases in the reverse-bias region, the velocity of the minority carriers responsible
for the reverse saturation current Io will also increase. Eventually, their velocity and
associated kinetic energy will be sufficient to release additional carriers through
collisions with otherwise stable atomic structures. That is, an ionization process will
result whereby valence electrons absorb sufficient energy to leave the parent atom.
These additional carriers can then aid the ionization process to the point where a
high avalanche current is established and the avalanche breakdown region
determined. The avalanche region (VZ) can be brought closer to the vertical axis by
increasing the doping levels in the p- and n-type materials. However, as VZ
decreases to very low levels, such as -5 V, another mechanism, called Zener
breakdown, will contribute to the sharp change in the characteristic. It occurs
because there is a strong electric field in the region of the junction that can disrupt
the bonding forces within the atom and generate carriers. Although the Zener
breakdown mechanism is a significant contributor only at lower levels of VZ, this
sharp change in the characteristic at any level is called the Zener region and diodes
employing this unique portion of the characteristic of a p-n junction are called Zener
diodes.
The maximum reverse-bias potential that can be applied before entering the Zener
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region is called the peak inverse voltage (referred to simply as the PIV rating) or the
peak reverse voltage (denoted by PRV rating). If an application requires a PIV rating
greater than that of a single unit, a number of diodes of the same characteristics can
be connected in series. Diodes are also connected in parallel to increase the current-
carrying capacity.
Zener breakdown region
ZENER DIODES
The characteristic drops in an almost vertical manner at a reverse-
bias potential denoted VZ. The fact that the curve drops down and
away from the horizontal axis rather than up and away for the
positive VD region reveals that the current in the Zener region has a
direction opposite to that of a forward-biased diode.
This region of unique characteristics is employed in the design of Zener diodes,
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which have the graphic symbol appearing in below figure. Both the semiconductor
diode and zener diode are presented side by side in the below figure, to ensure
that the direction of conduction of each is clearly understood together with the
required polarity of the applied voltage. For the semiconductor diode the on state
will support a current in the direction of the arrow in the symbol. The location of the
Zener region can be controlled by varying the doping levels. An increase in doping,
producing an increase in the number of added impurities, will decrease the Zener
potential. Zener diodes are available having Zener potentials of 1.8 to 200 V with
power ratings from 14 to 50 W. Because of its higher temperature and current
capability, silicon is usually preferred in the manufacture of Zener diodes.
.
The complete equivalent circuit of the Zener diode in the Zener region includes a
small dynamic resistance and dc battery equal to the Zener potential, as shown in
below figure
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7. Draw the circuit diagram and explain the working of full wave bridge rectifier and
derive the expression for average output current and rectification efficiency.
(AUC NOV’10, DEC 11)
A bridge rectifier makes use of four diodes in a bridge arrangement to achieve
full-wave rectification. This is a widely used configuration, both with individual
diodes wired as shown and with single component bridges where the diode
bridge is wired internally.
For both positive and negative swings of the transformer, there is a forward path
through the diode bridge. Both conduction paths cause current to flow in the same
direction through the load resistor, accomplishing full-wave rectification.
While one set of diodes is forward biased, the other set is reverse biased and
effectively eliminated from the circuit.
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8. Explain the operation of FWR with centre tap transformer. Also derive the Following
for this transformer. (AUC APR09, NOV10, DEC11)
(i)Dc output voltage
(ii) dc output current
(iii) RMS output voltage.
Full wave Rectifier using a centre tapped transformer:
A full-wave rectifier converts an ac voltage into a pulsating dc voltage using both
half cycles of the applied ac voltage. In order to rectify both the half cycles of ac input,
two diodes are used in this circuit. The diodes feed a common load RL with the help of
a center-tap transformer.
A center-tap transformer is the one which produces two sinusoidal waveforms of
same magnitude and frequency but out of phase with respect to the ground in the
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secondary winding of the transformer.
The full wave rectifier is shown in the figure below.
Voltage Waveforms of a center tapped full wave rectifier
Operation:
During the positive half cycle of the input, the anode of D1 becomes positive and the
anode of D2 becomes negative. Hence, D 1 conduct and D2 does not conducts. The
load current flows through D1 and the voltage drop across RL will be equal to the input
voltage.
During the negative half cycle of the input, the anode of D1 becomes
negative and the anode of D2 becomes positive. Hence, D 1 does not conduct and D2
conducts. The load current flows through D2 and the voltage drop across RL will be
equal to the input voltage.
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It is noted that the load current flows in the both the half cycles of ac voltage
and in the same direction through the load resistance.
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9. Explain the following regulator circuits: (AUC NOV’10)
(i)Transistor shunt regulator.
(ii) Zener diode shunt regulator.
10. (i)Distinguish between avalanche and zener mechanisms.
(ii)Explain how barrier potential is developed at the PN junction. (AUC APR’09)
(i) Avalanche & Zener mechanisms:
There is a point where the application of too negative a voltage will result in a sharp
change in the characteristics, as shown in the below figure. The current increases at
a very rapid rate in a direction opposite to that of the positive voltage region. The
reverse-bias potential that results in this dramatic change in characteristics is called
the Zener potential and is given the symbol VZ. As the voltage across the diode
increases in the reverse-bias region, the velocity of the minority carriers responsible
for the reverse saturation current Io will also increase. Eventually, their velocity and
associated kinetic energy will be sufficient to release additional carriers through
collisions with otherwise stable atomic structures. That is, an ionization process will
result whereby valence electrons absorb sufficient energy to leave the parent atom.
These additional carriers can then aid the ionization process to the point where a
high avalanche current is established and the avalanche breakdown region
determined. The avalanche region (VZ) can be brought closer to the vertical axis by
increasing the doping levels in the p- and n-type materials. However, as VZ
decreases to very low levels, such as -5 V, another mechanism, called Zener
breakdown, will contribute to the sharp change in the characteristic. It occurs
because there is a strong electric field in the region of the junction that can disrupt
the bonding forces within the atom and generate carriers. Although the Zener
breakdown mechanism is a significant contributor only at lower levels of VZ, this
sharp change in the characteristic at any level is called the Zener region and diodes
employing this unique portion of the characteristic of a p-n junction are called Zener
diodes.
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The maximum reverse-bias potential that can be applied before entering the Zener
region is called the peak inverse voltage (referred to simply as the PIV rating) or the
peak reverse voltage (denoted by PRV rating). If an application requires a PIV rating
greater than that of a single unit, a number of diodes of the same characteristics can
be connected in series. Diodes are also connected in parallel to increase the current-
carrying capacity.
Zener breakdown region
(ii) Barrier potential development across pn junction:
The semiconductor diode is formed by simply bringing these materials together
(constructed from the same base—Ge or Si). At the instant the two materials are
joined the electrons and holes in the region of the junction will combine, resulting in
a lack of carriers in the region near the junction. This region of uncovered positive
and negative ions is called the depletion region due to the depletion of carriers in
this region. Since the diode is a two-terminal device, the application of a voltage
across its terminals leaves three possibilities: no bias (VD = 0 V), forward bias (VD
>0 V), and reverse bias (VD< 0 V).
p-n junction with no external bias.
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Diode with no bias
Under no-bias (no applied voltage) conditions, any minority carriers (holes) in the n-
type material that find themselves within the depletion region will pass directly into
the p-type material. The closer the minority carrier is to the junction, the greater the
attraction for the layer of negative ions and the less the opposition of the positive ions
in the depletion region of the n-type material. Assume that all the minority carriers of
the n-type material that find themselves in the depletion region due to their random
motion will pass directly into the p-type material. Similar discussion can be applied to
the minority carriers (electrons) of the p-type material. This carrier flow has been
indicated in the above figure for the minority carriers of each material. The majority
carriers (electrons) of the n-type material must overcome the attractive forces of the
layer of positive ions in the n-type material and the shield of negative ions in the p-
type material to migrate into the area beyond the depletion region of the p-type
material. However, the number of majority carriers is so large in the n-type material
that there will invariably be a small number of majority carriers with sufficient kinetic
energy to pass through the depletion region into the p-type material. Again, the same
type of discussion can be applied to the majority carriers (holes) of the p-type
material. The resulting flow due to the majority carriers is also shown in the above
figure. In the absence of an applied bias voltage, the net flow of charge in any one
direction for a semiconductor diode is zero. The symbol for a diode is shown in the
below figure with the associated n- and p-type regions. Note that the arrow is
associated with the p-type component and the bar with the n-type region. As
indicated, for VD= 0 V, the current in any direction is 0 mA.
Reverse-Bias Condition (VD < 0 V)
If an external potential of V volts is applied across the p-n junction such that the
positive terminal is connected to the n-type material and the negative terminal is
connected to the p-type material as shown in the below figure. The number of
uncovered positive ions in the depletion region of the n-type material will increase
due to the large number of free electrons drawn to the positive potential of the
applied voltage. For similar reasons, the number of uncovered negative ions will
increase in the p-type material. The net effect, therefore, is a widening of the
depletion region. This widening of the depletion region will establish too great a
barrier for the majority carriers to overcome, effectively reducing the majority carrier
flow to zero as shown in the below figure.
SSathya Priyadharshini. Asst. Prof./EEE 27
Reverse-biased p-n junction.
The number of minority carriers, however, that find themselves entering the
depletion region will not change, resulting in minority-carrier flow vectors of the
same magnitude with no applied voltage the current that exists under reverse-bias
conditions is called the reverse saturation current and is represented by Io.
Forward-Bias Condition (VD > 0 V)
A forward-bias condition is established by applying the positive potential to the p-
type material and the negative potential to the n-type material as shown in the
below figure. A semiconductor diode is forward-biased when the association p-type
and positive and n-type and negative has been established.
Forward-biased p-n junction
The application of a forward-bias potential VD will pressure electrons in the n-type
material and holes in the p-type material to recombine with the ions near the
boundary and reduce the width of the depletion region as shown in the above
figure. The resulting minority-carrier flow of electrons from the p-type material to
the n-type material (and of holes from the n-type material to the p-type material)
has not changed in magnitude (since the conduction level is controlled primarily by
the limited number of impurities in the material), but the reduction in the width of
the depletion region has resulted in a heavy majority flow across the junction. An
SSathya Priyadharshini. Asst. Prof./EEE 28
electron of the n-type material now sees a reduced barrier at the junction due to
the reduced depletion region and a strong attraction for the positive potential
applied to the p-type material.
11. Discuss the following: (i) Transition capacitance (ii) Diffusion capacitance. (AUC MAY’08)
Transition & Diffusion Capacitance:
Electronic devices are inherently sensitive to very high frequencies. Most shunt
capacitive effects that can be ignored at lower frequencies because the reactance
XC=1/2πfC is very large (open-circuit equivalent). This, however, cannot be ignored
at very high frequencies. XC will become sufficiently small due to the high value of f to
introduce a low-reactance ―shorting‖ path. In the p-n semiconductor diode, there
are two capacitive effects to be considered. In the reverse-bias region we have the
transition- or depletion-region capacitance (CT), while in the forward-bias region we
have the diffusion (CD) or storage capacitance. The basic equation for the
capacitance of a parallel-plate capacitor is defined by C=€A/d, where € is the
permittivity of the dielectric (insulator) between the plates of area A separated by a
distance d. In the reverse-bias region there is a depletion region (free of carriers) that
behaves essentially like an insulator between the layers of opposite charge. Since
the depletion width (d) will increase with increased reverse-bias potential, the
resulting transition capacitance will decrease. The fact that the capacitance is
dependent on the applied reverse-bias potential has application in a number of
electronic systems. Although the effect described above will also be present in the
forward-bias region, it is overshadowed by a capacitance effect directly dependent on
the rate at which charge is injected into the regions just outside the depletion region.
The capacitive effects described above are represented by a capacitor in parallel with
the ideal diode, as shown in the below figure. For low- or mid-frequency applications
(except in the power area), however, the capacitor is normally not included in the
diode symbol.
SSathya Priyadharshini. Asst. Prof./EEE 29