BEEME UNIT IV.ppt

UNIT IV
SEMICONDUCTOR DEVICES AND
APPLICATIONS
BASIC ELECTRICAL,
ELECTRONICS AND
MEASUREMENTS ENGINEERING

INTRODUCTION ABOUT
SEMICONDUCTOR
 The most fundamental unit of all matter is an
atom
 An element or the matter consists of three
fundamental particles. These three particles are
electron, proton and neutron.
 Electron is a negatively charged particle. It is
denoted as 'e-'.
 Proton is a positively charged particle. It is denoted
as 'p+'.
 Neutron is an uncharged particle.
 Most solid materials are classed, from the
standard point of electrical conductivity, as
conductors, semiconductors and insulators.

Energy Band
 In Energy band, only two upper band of energy
levels are considered.
 The upper most band is conduction band.
 The lower most band is valence band.
 These two bands are separated by a gap. This
gap is known as forbidden energy gap.

 All electrons in the conduction band are free
electrons.
 In Valance Band, not free to move about as like
electrons in the conduction band.
 Electrons may jump back and forth from the
bottom valence band to the upper conduction
band. But, they never come to rest in the
forbidden energy gap.
Classification of Solids
Based on the band theory, solids are
classified into three types. They are
i) Conductors
ii) Insulators
iii) Semiconductors

Conductors
 The conductor's atoms have only one valence
electrons.
 These electrons are very loosely bound with nucleus, so
it can easily break away from their atom and become
free electron
 Here there is no forbidden energy gap between the
conduction band and valance band.
 The resistance of the conductor is very small. Example:
copper, aluminum

Insulators
 The energy gap between valence band and conduction
band is very large.
 Eight valence electrons are bound very tightly to parent
atoms, thus requiring very large electric field to remove
these electrons from bonding to their nucleus.
 If the temperature is raised, some of the valence
electrons may acquire energy to move to the conduction
band. Thus, the resistivity decreases. Example: Paper,
Mica, Sodium chloride.

Semiconductors
 The energy gap between valence band and conduction
band is very small. Electrical properties lie in between
that of insulators and conductors.
 A semiconductor virtually behaves as an insulator at
low temperature.
 When the temperature is increased width of forbidden
energy band is decreased so that some of electrons
are liberated into conduction band. At these condition
semiconductors act as a conductor. Example: Silicon,
Germanium, Gallium arsenide etc.

Types of Semiconductors
 Semiconductors
 Intrinsic semiconductor
 Extrinsic semiconductor
 N-type semiconductor
 P-type semiconductor

Intrinsic Semiconductor
 Intrinsic semiconductor is a pure semiconductor.
 The Si and Ge atoms contain 4 electrons in the
outermost orbit. So they are called as tetravalent atoms.
 The co-valent bond structure of Ge atom is shown in
Figure

 Behaves as a perfect insulator at low temperature
(0°K). Now no electrons get away from the co-valent
bond. So the current flow is zero.
 At room temperature, some of the valence elements
may acquire sufficient energy. The bonds may be
broken

 When an external electric field is applied across
intrinsic semiconductor, more number of electron-hole
pair combinations will be broken.
 According to the amount of electric field, many free
electrons are moved to the positive potential through
holes, called electron current.
 Now the holes are moved towards the negative
potential called hole current.
 The sum of electron current and hole current is known
as electric current.

Extrinsic semiconductors
 The electrical conductivity of pure semiconductor is
increased by adding some impurities in it. The
resultant semiconductor is called extrinsic
semiconductor.
 The process of adding impurities to a pure
semiconductor is known as doping.
 The purpose of adding impurities in the pure
semiconductor is to increase the number of free
electrons or holes, for increasing the conductivity.

1. N-type semiconductor
 Adding a small amount of penta valent impurities to a
semiconductor material.
 The added impurities are called as donor impurities
because they will donate electrons.
 The 4 valence electrons of antimony atom form co-
valent bonds with 4 valence electrons of individual Ge
atom.
 The 5th valence electron of antimony is left free, loosely
bound to the antimony atom.
 This loosely bound electron can be easily excited from
the valence band to the conduction band by the
application of small electric field.

 Semiconductor contains more electrons and less
holes. Hence it is called N-type semiconductor.
 So the electrons are called majority carrier and
holes are minority carriers.

2. P-Type Semiconductor
 Adding a small amount of trivalent impurities (such as
Aluminium or Boron) to a pure semiconductor (such as
Silicon or Germanium) material.
 Three valence electrons in aluminium form co-valent
bond with four surrounding atoms of Ge. Now one co-
valent bond is incomplete which give rise to a hole.

 The holes increase the conductivity of the P-type
semiconductor. The impurities are known as acceptor
impurities, because the holes created can accept
electrons.
 The number of holes is more than the number of
electrons.
 In P-type semiconductors holes are majority carriers
and electrons are minority carriers.

PN JUNCTION DIODE
 Join a piece of P-type semiconductor to a piece of N-
type semiconductor such that the crystal remains
contionues at the boundary.
 PN junction forms very useful device and is called a
seiconductor diode or PN junction diode as shown
Holes and Electrons – mobile charge carriers
Positive and negative ions – immobile charges

Formation of Depletion Layer in a
PN junction
 In P-region has holes(Majority carrier) and negatively
charged impurity atoms, called negative ions (acceptor
ions)
 In N-region has free electrons(Majority carrier) and
positively charged impurity atoms, called negative ions
(donor ions)
 Holes and Electrons are the mobile charge carriers.
Positive and Negative ions are immobile charges and it
do not in conduction.
 As soon as the PN junction is formed, some of the holes
in P-region and free electrons in N-region diffuse each
other and disappear due to recombination.

Formation of Depletion Layer in a PN
junction

V-I characteristics under forward
biased condition
Cut in voltage (V0) 0.3 v for Germanium
0.7 v for silicon
At cut in voltage, potential barrier is overcome and current through
the junction starts increases rapidly.

 When positive terminal of the battery is connected to
the P-type and negative terminal to the N-type of the PN
junction diode, the bias is known as forward bias.
 Applied positive potential repels the holes in P-type
region so that holes move towards the junction and the
applied negative potential repels the electrons in the N-
type region and the electrons moves towards the
junction.
 When applied potential is more then disappear
depletion region.
 When VF<V0 then forward current almost zero.
 When VF>V0 then large forward current flows. Here
potential barrier or depletion layer is disappear.

V-I characteristics under reverse
biased condition

 When negative terminal of the battery is connected to the P-
type and positive terminal to the N-type of the PN junction
diode, the bias is known as reverse bias.
 Under reverse bias, holes of majority carrier in P-type region
move towards negative terminal of the battery and electrons
of majority carrier in N-type region move towards positive
terminal of the battery.
 Which is increases the depletion layer or potential barrier.
 Ideally there is no current flows. Practically very small current
of order of few microampere flows.
 Minority carrier in P-region and N-region trying to flows
across junction and give rise to small reverse current. This
current known as Reverse saturation current.
 When large reverse voltage applied then sufficient energy to
dislodge valance electron. Then conduction takes place and
which voltage called breakdown voltage

Ideal Diode
 The first electronic device to be introduced is called the
diode. It is the simplest of semiconductor devices but
plays a very vital role in electronic systems, having
characteristics that closely match those of a simple
switch. It will appear in a range of applications,
extending from the simple to the very complex.
 In addition to the details of its construction and
characteristics, the very important data and graphs to
be found on specification sheets will also be covered to
ensure an understanding of the terminology employed
and to demonstrate the wealth of information typically
available from manufacturers.

 The term ideal will be used frequently in this text as
new devices are introduced. It refers to any device or
system that has ideal characteristics—perfect in every
way.
 It provides a basis for comparison, and it reveals
where improvements can still be made. The ideal
diode is a two-terminal device having the symbol and
characteristics shown in figures

 Ideally, a diode will conduct current in the direction
defined by the arrow in the symbol and act like an open
circuit to any attempt to establish current in the
opposite direction.
 The characteristics of an ideal diode are those of a
switch that can conduct current in only one direction.
 One of the important parameters for the diode is the
resistance at the point or region of operation.
 If we consider the conduction region defined by the
direction of ID and polarity of VD in figure. (upper-right
quadrant of figure.), we will find that the value of the
forward resistance, RF, as defined by Ohm’s law is

 Consider the region of negatively applied potential (third
quadrant) of figure.

JUNCTION DIODE SWITCHING
CHARACTERISTICS
 Diodes are often used in a switching mode. When the
applied bias voltage to the PN junction diode is suddenly
reversed in the opposite direction, the diode response
reaches a steady state after an interval of time, called
the recovery time.
 The forward recovery time, tf is defined as the time
required for forward voltage or current to reach a
specified value (time interval between the instant of 10%
diode voltage to the instant this voltage reaches within
10% of its final value) after switching diode from its
reverse to forward biased state.
 Fortunately, the forward recovery time posses no series
problem. Therefore, only the reverse recovery time, trr
has to be considered in practical applications.

 When the PN junction is forward biased, the minority
electron concentration in the P-region is approximately
linear. If the junction is suddenly reverse biased, at t1,
then because of this stored electronic charges, the
reverse current (IR) is initially of the same magnitude as
the forward current (IF).
 The diode will continue to conduct until the injected or
excess carrier minority density (p=-po) or (n-no) has
dropped to zero.
 However, as the stored electrons are removed into the
N-region and the contact, the available charge quickly
drops to an equilibrium level and a steady current
eventually flows corresponding to the reverse bias
voltage as shown in figure (c).
 As shown in figure (b), the applied voltage Vi=VF for the
time up to t1 is in the direction to forward bias the diode.

 Then the current is I= . Then at time t= t1, the input
voltage is suddenly reversed to the value of –VR.
 Due to the reason, the current does not become zero
and has the value I= until the time t=t2.
 At t= t2, when the excess minority carriers have reached
the equilibrium state, the magnitude of the diode current
starts to decreases as shown in figure (d).
 During the time interval from t1 to t2, the injected minority
carriers have remained stored and hence this interval is
called the storage time (ts).
 After the instant t= t2 the diode gradually recovers and
ultimately reaches the steady state.

 The time interval between t2 and the instant t3 when the
diode has recovered nominally is called the transition
time, tr.
 The recovery is said to have completed
 (i) when even the minority carriers remote from the
junction have diffused to the junction and crossed it, and
 (ii) when the junction transition capacitance, CT across the
reverse biased junction has got charged through the
external resistor RL to the voltage -VR.
 The reverse recovery time (or turn off time) of a diode,
trr is the interval from the current reversal at t= t1 until
the diode has recovered to a specified extent in terms
either of the diode current or of the diode resistance i.e
trr= ts + tr
 For a commercial switching type diodes the reverse
recovery time trr, ranges from 1 ns up to as high as 1μs.

 If the time period of the input signal is such that T =2.trr,
then the diode conducts as much in reverse as in the
forward direction. Hence it does not behave as a one
way device.
 In order to minimize the effect of the reverse current,
the time period of the operating frequency should be a
minimum of approximately 10 times trr. For example, if a
diode has trr of 2ns, its maximum operating frequency is
 The trr, can be reduced by shortening the length of the
P-region in a PN junction diode.
 The stored charge and consequently the switching time
can also be reduced by introduction of gold impurities
into the junction diode by diffusion.

ZENER DIODE
 Zener diode is a reverse biased heavily doped PN
junction diode which operates in breakdown region.
 The reverse breakdown of a PN junction may occurs
either due to zener effect or avalanche effect.
 Zener effect dominates at reverse voltage less than 6V
and avalanche effect dominates above 6V
 For zener diodes, Silicon is preferred to Ge because of
its higher temperature and current capability.
 Symbol of zener diode as shown

Forward biasing zener diode
 Anode connected to positive terminal of battery and
cathode connected to negative terminal of battery.
 Its behavior identical to F.B diode
 General zener diode not used in F.B condition

Reverse biasing zener diode
 Cathode connected to positive terminal of battery and
Anode connected to negative terminal of battery.
 Its operation is differ from that of diode.
 Zener diode in reverse biased condition is used as a
voltage regulator.

V-I characteristics of zener diode
 V-I characteristics of zener diode can be divided into
two parts
 Forward characteristics
 Reverse characteristics
 Forward characteristics
 The characteristics as shown
 It is almost identical to the as a PN junction diode

 Reverse characteristics
 Reverse voltage increases, initially small reverse saturation current I0, in order
of μA will flow. This current due to thermally generated minority carriers.
 At particular reverse voltage, reverse current increase sharp and suddenly.
This indication that breakdown occurs.
 This breakdown voltage is called as zener breakdown voltage or zener voltage
and it is denoted by Vz
 After breakdown Vz remains constant and further increase only reverse zener
current.
 For controlling zener current put R and which avoid excess heat.

Application
 Zener diode is used as a voltage regulator
 Zener diode is used as a peak clipper in wave
shaping circuits
 Zener diode is used as a fixed reference voltage
in transistor biasing circuits.
 Zener diode is used for meter protection against
damage from accidental application of excessive
voltage.

Breakdown mechanism
 If reverse bias voltage applied to a PN junction is
increased, a point will reach when the junction
breakdown and reverse current rises sharply to a value
limited only by the external resistance connected in
series.
 This specific value of reverse bias voltage is called
breakdown voltage
 The breakdown voltage depends on width of depletion
layer. This width of depletion layer depends on doping
level.
 Process of causes junction breakdown due to increase
in reverse bias voltage as
 Zener breakdown
 Avalanche breakdown

Zener breakdown
 It observed when Vz<6V. If apply Vz then strong electric
field appear across narrow depletion region.
 Value of electric field as 3*10^5v/cm.
 Due to this electric field pull valance electron into
conduction band to breaking covalent bond.
 So large no of free electron causes to reverse current
through zener diode and breakdown occurs due to
zener effect.

Avalanche Breakdown
 It observed when Vz>6V.
 Reverse bias condition conduction due to only in
minority carrier.
 Reverse voltage increase, then accelerates minority
carrier and causes to increase K.E
 Accelerates minority carrier collide with stationary atom
and K.E causes valance electron present in covalent
bond.
 Now valance electron breakdown covalent bond and
become free for conduction.
 Now increase more no free electrons collide. This
phenomenon is called as avalanche multiplication.

 In short time large no of free minority electrons and
holes available for conduction and which causes self
sustained multiplication process called ‘Avalanche
effect’
 Large reverse current starts flowing through zener diode
and occur avalanche breakdown.

RECTIFIER
 Rectifier is defined as an electronic devices used for
converting a.c voltage into unidirectional voltage.
 A rectifier utilizes unidirectional conduction device like a
PN junction diode.
 Rectifiers are classified depending upon the period of
conduction as
 Half –wave rectifier
 Full –wave rectifier

Half wave Rectifier
 The circuit diagram and its waveform as shown. It
conduct only during positive half cycle

Operation
 The AC voltage across secondary winding AB changes
polarities after every half cycle.
 During +ve half cycle of input AC voltage end A
becomes positive w.r.to B. This makes the diode
forward biased and hence it conducts current.
 During –ve half cycle end A is negative w.r.to end B.
Under this condition, the diode is reverse biased and
hence it not conduct.
 Therefore current flows through the diode during +ve
half cycle of input AC voltage only.

Ripple Factor
 The unwanted AC component present in the DC
output are called as ripple. Ripples can be removed
by using filter circuits.
 Ripple factor is defined as the ratio of the effective
value or rms value of the ac component of the voltage
or current to the average value of voltage or current.

Regulation
 The variation of dc output voltage with change in D.C
load current is defined as the regulation. The percentage
regulation is calculated as follows.
 Percentage regulation =
 The lower the percentage regulation, the better would be
the power supply. An ideal power supply will have a zero
percentage regulation.
 Average or dc value of voltage across the load is given
as

Transformer Utilization Factor
(TUF)

 Advantages
 It is simple
 Low cost
 Disadvantages
 Low rectification efficiency
 High ripple factor
 Low TUF
 Since current flows for only one half cycle core
saturation result.

Full Wave Rectifier
 FWR is a circuit which allows a unidirectional current
to flow through the load during the entire input cycle.
 There are two types of full wave rectifiers as
 Center tapped full wave rectifier
 Bridge rectifier

Center tapped FWR
 In full wave rectification, current flows through the load in
the same direction for both half cycles of the input AC
voltage.
 It uses the center tapped transformer which provides
equal voltages above and below the center tapped for
both half cycles.
 The voltage between the center tap and either end of the
secondary winding is half of the secondary voltage.
 The center tap of the secondary winding of a transformer
is taken as the ground or zero voltage reference point.

 The circuit uses two diodes, which are connected
to the center tapped secondary winding of the
transformer as shown
 Diode D1 utilizes the AC voltage appearing
across the upper half (OA) of secondary winding
for rectification while diode D2 uses the lower half
winding (OB).

Ripple Factor
 It is defined as the ratio of the rms value of a.c
voltage to the d.c voltage.

Peak Inverse Voltage (PIV)
 It is defined as the maximum voltage that a diode can
withstand under reverse biased condition.
 In this case PIV is calculated as follows.
 Assume during positive half cycle of input D1 is
conducting and D2 is off. The maximum voltage at the
lower part of the transformer is Vm and the voltage drop
across the RL due to diode D1 conducting is Vm.
 Hence the total voltage (across diode D2 is 2Vm) (i.e) in
lower part of the transformer is Vm+Vm=2Vm.
 This is the voltage applied across diode D2
 PIV=2Vm
 The same procedure is repeated when D1 if OFF and D2 is
ON.

Transformer Utilization Factor
(TUF)
 In this case TUF is found by considering primary and
secondary VA rating separately and take the average of
two values.
 TUF for secondary can be calculated as (because for
each half cycle of input only one half of the transformer
secondary is effectively used)

 Advantages
 The output voltage and transformer efficiency are
higher
 The dc saturation of the core is avoided as current
flows through the two halves of the center tapped
secondary of the transformer
 Lower ripple factor
 Higher TUF
 Disadvantages
 Usage of additional diode and bulky transformer is
needed, and hence increase in cost
 PIV of diode is high
 The output voltage is half of the secondary voltage.

Full wave Bridge Rectifier
 The need for a center tapped power transformer is
eliminated in the bridge rectifier, as shown

 It contains 4 diodes D1, D2, D3, D4 connected to form
bridge.
 The ac supply to be rectified is applied to the diagonally
opposite ends of the bridge through the transformer.
Between other two ends of the bridge the load
resistance RL is connected.
Input Output Waveform as shown:

 The derivative of bridge rectifiers are same as that of
center tapped FWR, except TUF

 Advantages
 The need for center tapped transformer is eliminated
 The output is twice that of the center tap circuit for the
same secondary voltage
 The PIV is one half that of the center tap circuit
 Disadvantages
 It requires four diode
 As during each half cycle of a.c input two diodes that
conduct are in series, therefore, voltage drop in the
internal resistance of the rectifying unit will be twice as
great as in the center tap circuit.

NPN TRANSISTOR PNP TRANSISTOR
WORKING OF TRANSISTOR

TRANSISTOR CONSTRUCTION
 Transistor construction
 Types as n-p-n and p-n-p transistor
 Three region as
 Emitter (E)
 Base (B)
 Collector (C)
 Emitter
 It placed one side, which supplies charge carrier to other
region. It is heavily doped
 Base
 It is middle in region forms two PN junction. It is lightly
doped and thinner region
 Collector
 It is opposite to the emitter and collect charge. It is larger
region compared to other two. Doping level is intermediate.

Standard transistor symbol
 It has two junction like two diode
 JE as emitter diode and act as F.B
 JC as collector diode and act as R.B

Modes of operation
 If transistor operate as an amplifier, it biased with
external voltages.
 Depends on external bias voltage polarity, transistor
works as any one of the three region

Transistor operation
 E-B junction as F.B and C-B junction as R.B.
 Ie flows through the collector region.
 Working as npn transistor:
 E-B junction as F.B and C-B junction as R.B
 Electrons in n-region(E) flows in to p-region(B)
 B- lightly doping so that small electrons only recombine to
holes and remaining current flows to n-region (C)
 IE=IB+IC
 IC has minority carrier and majority carrier current

 Working as pnp transistor:
 E-B junction as F.B and C-B junction as R.B
 Holes in p-region(E) flows in to n-region(B)
 B- lightly doping so that small holes only recombine to
electrons and remaining holes current flows to p-region
(C)
 Current conduction takes place only by holes
 But external wiring still current be electron.

Types of configuration
 Connection of transistor circuit as
 One terminal is i/p
 Other terminal is o/p
 Other terminal common between i/p and o/p
S.No Configuration Input Output Common
terminal
Also Called as
1 CB E C B Grounded Base
Configuration
2 CE B C E Grounded Emitter
Configuration
3 CC B E C Grounded
Collector
Configuration

Common Base configuration
 I/p applied as E-B
 O/p taken as C-B
 Base is common for i/p and o/p

Common Base Characteristics
 I/p characteristics
 It relates as i/p current (IE) with i/p voltage (VBE) for varies
output voltage (VCB)

 Observation as
 Point 1
 IE ↑ rapidly after cut in voltage with small ↑VBE
 Indicate Rin is small.
 Dynamic i/p resistance as
 Point 2
 ↑VCB then slight ↑IE
 Reason as VCB ↑, then width of depletion region (Jc
junction) changed under R.B condition.
 Depletion width in Je junction small due to F.B

 O/p Characteristics
 It relates as o/p current (IC) with o/p voltage (VCB) for varies
input current (IE). In diagram Y-axis as IC (wrong)

 Observation as
 Active region
 IC ↑ with equal to IE. IC independent on VCB
 Relationship as
 B-E jn as F.B and C-B jn as R.B
 Cut-off region
 IE=0, IC=ICBO due to reverse saturation current.
 ICBO as on order of μA or nA. Here IC≈0
 B-E jn and C-B jn both as R.B

 Saturation region
 This is the region as left of VCB=0
 B-E jn and C-B jn both as F.B
 Early effect or base width modulation
 VC ↑, R.B ↑, then (depletion width between C-B)↑
 As result in base width ↓
 Causes of base width ↓ as
 Less chances of recombination in B- region, then α ↑ with VCB ↑
 Charge gradient ↑ in B-region, minority carrier I injected in E-region ↑
 Large VC, B-width ↓ effectively (may be zero), causes B.D voltage in
transistor. This phenomenon called punch through.

Common Emitter Configuration
 I/p applied as B-E and o/p applied as C-E
 E is common for both i/p and o/p

Common Emitter Characteristics
 Relates i/p current (IB) to i/p voltage (VBE) for varies level
of o/p voltage (VCE) (here Y-axis is IB)

 Observation as
 Point 1
 IB ↑ rapidly after cut-in voltage of VBE.
 So Rin is small.
 Point 2
 VCE ↑, IB ↓, here VBE=const
 Reason of IB ↓ as R.B ↑ at Jc junction which means C-B
junction.
 Then depletion layer width ↑
 Width of B-region ↓ and ↓ of recombination in B-region

 It relates as o/p current (IC) with o/p voltage (VCE) for varies
input current (IB).

 Observation as
 Active region
 B-E as F.B and C-B as R.B
 VCE ↑, then R.B ↑. Due to Early effect width of depletion
region ↑ and base width ↓ then recombination in base↑
 Small change in α result large change in β
 Compare to CB config when CE config o/p char slope is
large
 Cut-off region
 B-E and C-B both as R.B
 IB=0, IC=ICEO (reverse leakage current) which is very small.

 The region below IB=0 is known as cut-off region
 Saturation region
 B-E and C-B both as F.B
 Region to left of VCE(sat) is called the saturation region.
 VCE ↓ very less, then C-B junction as become F.B
 Ranges of VCE as 0.1 to 0.3V

Common collector
configuration
 I/p applied as B-C and o/p taken as E-C
 C is common for both i/p and o/p

Characteristics
 Relates i/p current (IB) to i/p voltage (VBC) for varies level of
o/p voltage (VEC)

 I/p Char of CC config as differ from CE and CB config.
The reason as follows
 VBC ↑, with VEC=const, then VEB↓ and IB ↓
 Which explain slope of CC i/p char.

 It relates as o/p current (IE) with o/p voltage (VCE) for
varies input current (IB).
 Its char is similar to CE configuration.

Comparison of CE, CB and CC
configuration

LIGHT EMITTING DIODE
 The increasing use of digital displays in calculators,
watches, and all forms of instrumentation has
contributed to the current extensive interest in
structures that will emit light when properly biased.
 The two types in common use today to perform this
function are the light-emitting diode (LED) and the
liquid-crystal display (LCD).
 As the name implies, the light-emitting diode (LED) is a
diode that will give off visible light when it is energized.
 In any forward-biased p-n junction there is, within the
structure and primarily close to the junction, a
recombination of holes and electrons.
 This recombination requires that the energy possessed

 In all semiconductor p-n junctions some of this energy
will be given off as heat and some in the form of photons.
In silicon and germanium the greater percentage is given
up in the form of heat and the emitted light is
insignificant.
 In other materials, such as gallium arsenide phosphide
(GaAsP) or gallium phosphide (GaP), the number of
photons of light energy emitted is sufficient to create a
very visible light source.
 The process of giving off light by applying an electrical
source of energy is called electroluminescence.

Process of electroluminescence in
the LED and its Graphic symbol

 The conducting surface connected to the p-material is
much smaller, to permit the emergence of the maximum
number of photons of light energy.
 Note in the figure that the recombination of the injected
carriers due to the forward-biased junction results in
emitted light at the site of recombination.
 There may, of course, be some absorption of the
packages of photon energy in the structure itself, but a
very large percentage are able to leave, as shown in the
figure.

LIQUID-CRYSTAL DISPLAYS
 The liquid-crystal display (LCD) has the distinct
advantage of having a lower power requirement than
the LED.
 It is typically in the order of microwatts for the display,
as compared to the same order of milliwatts for LEDs.
 It does, however, require an external or internal light
source and is limited to a temperature range of about
0° to 60°C. Lifetime is an area of concern because
LCDs can chemically degrade.
 The types receiving the major interest today are the
field-effect and dynamic-scattering units.
 A liquid crystal is a material (normally organic for LCDs)
that will flow like a liquid but whose molecular structure

 For the light-scattering units, the greatest interest is in
the nematic liquid crystal, having the crystal structure
shown in figure
 The individual molecules have a rod like appearance as
shown in the figure.
 The indium oxide conducting surface is transparent, and
under the condition shown in the figure, the incident
light will simply pass through and the liquid-crystal
structure will appear clear.

 If a voltage (for commercial units the threshold level is
usually between 6 and 20 V) is applied across the
conducting surfaces, as shown in figure.
 The molecular arrangement is disturbed, with the result
that regions will be established with different indices of
refraction.
 The incident light is therefore reflected in different
directions at the interface between regions of different
indices of refraction with the result that the scattered

 A digit on an LCD display may have the segment
appearance shown in figure
 If the number 2 were required, the terminals 8,7, 3, 4,
and 5 would be energized, and only those regions
would be frosted while the other areas would remain
clear.

 The field-effect or twisted nematic LCD has the same
segment appearance and thin layer of encapsulated
liquid crystal, but its mode of operation is very different.
 Similar to the dynamic-scattering LCD, the field-effect
LCD can be operated in the reflective or transmissive
mode with an internal source. The transmissive display
appears in figure.

 The reflective-type field-effect LCD is shown in figure.
 In this case, the horizontally polarized light at the far left
encounters a horizontally polarized filter and passes
through to the reflector, where it is reflected back into
the liquid crystal, bent back to the other vertical
polarization, and returned to the observer. If there is no
applied voltage, there is a uniformly lit display. The
application of a voltage results in a vertically incident
light encountering a horizontally polarized filter at the

POWER CONDITIONING
EQUIPMENTS
 Power conditioning equipment is highly recommended
since unplanned disturbances on the electric utility's
system will occur.
 The two categories of power conditioning equipment
are "power enhancers" and "power synthesizers."
 Some manufacturers and suppliers loosely employ the
terms "power conditioner" and "line conditioner.“
 It provide more than one mode of power protection.

Power Enhancers
 Power enhancers provide a way to improve your
facility's electrical supply.
 However, power enhancers provide no help for
loss of power during a power outage.
 Some examples of power enhancers are:
 Surge Suppressors
 Voltage Regulators
 Isolation Transformers

Power Synthesizers
 Power synthesizers are capable of not only enhancing
the incoming power, but also providing auxiliary power
during utility outages.
 Power synthesizers are usually less efficient and
require more maintenance.
 Power Synthesizers include:
 Motor Generators
 Standby Power Supply
 Uninterruptible Power Supply
 Uninterruptible Power Supply with Auxiliary Generator

Linear Power Supply (LPS)
 The Linear Power Supply LPS is the regulated power
supply which dissipates much heat in the series
resistor to regulate the output voltage which has low
ripple and low noise.
 A linear power supply requires larger semiconductor
devices and generates more heat resulting in lower
energy efficiency
 As the electrical noise is lower, the LPS is used in
powering sensitive analog circuitry. But to overcome
the disadvantages of Linear Power Supply system,

 Advantages of LPS
 The power supply is continuous.
 Simple and reliable systems.
 This system dynamically responds to load changes.
 The circuit resistances are changed to regulate the output
voltage.
 The noise is low.
 The ripple is very low in the output voltage.
 Disadvantages of LPS
 The transformers used are heavier and large.
 The heat dissipation is more.
 The efficiency of linear power supply is 40 to 50%
 Power is wasted in the form of heat in LPS circuits.
 Single output voltage is obtained.

Switched Mode Power
Supply (SMPS)
 The working of SMPS is simply understood by knowing
that the transistor used in LPS is used to control the
voltage drop while the transistor in SMPS is used as
a controlled switch.

happens at each stage of SMPS
circuit
 Input Stage
 The AC input supply signal 50 Hz is given directly without
using any transformer.
 This output will have many variations and unregulated dc
is given to the central switching section of SMPS
 Switching Section
 Power transistor or a MOSFET is employed in this
section
 Which switches ON and OFF according to the variations
and this output is given to the primary of the transformer
present in this section.

 Output Stage
 The output signal from the switching section is again
rectified and filtered, to get the required DC voltage.
 This is a regulated output voltage which is then given to
the control circuit, which is a feedback circuit.
 The final output is obtained after considering the feedback
signal.
 Control Unit

 The output sensor senses the signal.
 The signal is isolated from the other section so that
any sudden spikes should not affect the circuitry.
 A reference voltage along with the signal to the error
amplifier which is a comparator that compares the
signal with the required signal level.
 By controlling the chopping frequency the final
voltage level is maintained.
 Error amplifier output helps to decide whether to
increase or decrease the chopping frequency.
 The PWM oscillator produces a standard PWM wave
fixed frequency.

 Advantages
 The efficiency is as high as 80 to 90%
 Less heat generation; less power wastage.
 Reduced harmonic feedback into the supply mains.
 The device is compact and small in size.
 The manufacturing cost is reduced.
 Provision for providing the required number of voltages.
 Disadvantages
 The noise is present due to high frequency switching.
 The circuit is complex.
 It produces electromagnetic interference.

 Applications
 computers,
 mobile phone chargers,
 HVDC measurements,
 battery chargers,
 central power distribution,
 motor vehicles,
 consumer electronics,
 laptops,
 security systems,
 space stations,

Uninterruptable Power Supply
(UPS)
 An Uninterruptible Power Supply (UPS) is defined as a
piece of electrical equipment which can be used as an
immediate power source to the connected load when
there is any failure in the main input power source.
 When there is any failure in main power source, the UPS
will supply the power for a short time. This is the prime
role of UPS.
 In addition to that, it can also able to correct some
general power problems related to utility services in
varying degrees.

Types of UPS
 On-line UPS,
 Off- line UPS and
 Line interactive UPS.
 Other designs include
 Standby on-line hybrid,
 Standby-Ferro,
 Delta conversion On-Line.

Off-line UPS
 This UPS is also called as Standby UPS system
 Here, the primary source is the filtered AC mains.
 When the power breakage occurs, the transfer switch
will select the backup source
 In this system, the AC voltage is first rectified and stored
in the storage battery connected to the rectifier. The
transfer time can be about 25 milliseconds

On-line UPS
 In this type of UPS, double conversion method is used
 Here, first the AC input is converted into DC by
rectifying process for storing it in the rechargeable
battery.
 This DC is converted into AC by the process of
inversion and given to the load or equipment which it is
connected
 This type of UPS is used where electrical isolation is
mandatory.

 When there is any power failure, the rectifier have no
role in the circuit
 Once the power is restored, the rectifier begins to
charge the batteries.
 To prevent the batteries from overheating due to the
high power rectifier, the charging current is limited.
 During a main power breakdown, this UPS system
operates with zero transfer time.
 But the presence of inrush current and large load
step current can result in a transfer time of about 4-6
milliseconds in this system.

Line Interactive UPS
 For small business and departmental servers and
webs, line interactive UPS is used. This is more or less
same as that of off-line UPS
 The difference is the addition of tap changing
transformer.
 Voltage regulation is done by this tap-changing
transformer by changing the tap depending on input
voltage.
 Additional filtering is provided in this UPS result in
lower transient loss.

 UPS Applications
 Data Centers
 Industries
 Telecommunications
 Hospitals
 Banks and insurance
 Some special projects (events)

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