Download CHAPTER 4: ELECTRICAL and ELECTRONIC PROPERTIES

7/30/2007
CHAPTER 4:
ELECTRICAL and ELECTRONIC
PROPERTIES
• How electrical conductance and resistance
are characterized.
• What physical phenomena distinguish
conductors, semiconductors, and insulators?
• For metals, how is conductivity affected by
imperfections, T, and deformation?
• For semiconductors, how is conductivity affected
by impurities (doping) and T?
1
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Alkali Metals
The Periodic Table
Alkaline Earths
Halogens
Noble Gases
Main Group
Transition Metals
Main Group
Lanthanides and Actinides
Periodic Properties of the Elements
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Electrical Conduction
• Ohm's Law:
∆V = I R
voltage drop (volts)
resistance (Ohms)
current (amps)
A
(cross
sect.
area)
e-
I
∆V
L
• Resistivity, ρ and Conductivity, σ:
--geometry-independent forms of Ohm's Law
E: electric
field
intensity
∆V I
= ρ
L
A
ρL
L
• Resistance: R =
=
A Aσ
resistivity
(Ohm-m)
J: current density
conductivity
σ=
I
ρ
3
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Effect of Impurity on Electrical
Conductivity
Metals are fairly insensitive to impurities. It takes
a lot of impurity to change the conductivity of a
metal by as much as a factor of 10. Unlike
semiconductors, metals become poorer conductors
when impure.
Semiconductors , on the other hand, are very
sensitive to impurities. The conductivity of silicon
or germanium can be increased by a factor of up to
106 by adding as little as 0.01% of an impurity.
• Problem 12.2, p. 524, Callister 2e:
e-
Cu wire -
100m
I = 2.5A
+
∆V
What is the minimum diameter (D) of the wire so that
∆V < 1.5V?
100m
< 1.5V
R=
2
πD
4
L
∆V
=
Aσ
I
2.5A
7
-1
6.07 x 10 (Ohm-m)
Solve to get D > 1.88 mm
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Conductivity Comparison
-1
• Room T values (Ohm-m)
METALS
conductors
Silver
6.8 x 107
Copper
6.0 x 107
Iron
1.0 x 107
CERAMICS
Soda-lime glass 10 -10
Concrete
10-9
Aluminum oxide <10-13
SEMICONDUCTORS
POLYMERS
-4
Polystyrene
<10-14
Silicon
4 x 10
Polyethylene 10-15-10 -17
Germanium 2 x 100
GaAs
10-6
insulators
semiconductors
Selected values from Tables 18.1, 18.2, and 18.3, Callister 6e.
4
Metals, Insulators and Semiconductors
METALS: Electron density is such that
valence band ( highest occupied band) is
partially (say, half) filled.
INSULATORS/SEMICONDUCTORS:
Valence band (highest occupied band) is
completely filled at low temperature.
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Energy Bands
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under
license.
The energy levels broaden into bands as the number of electrons
grouped together increases.
THE VALENCE BAND:The highest
energy band that is occupied by one or
more electrons is called the valence band.
THE CONDUCTION BAND:
The next higher vacant level is called the
conduction
band.
Band Theory
CONDUCTION BAND
BAND GAP
BAND GAP
VALENCE BAND
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Bonding in Metals
Electron sea model
Nuclei
in a sea of e-
.
Metallic lustre.
Malleability.
Force
applied
Bonding in Metals
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Bonding in Metals
Band theory.
According to band theory, the electrons in a crystal become free
to move when they are excited to the unoccupied orbitals of a
band.
–N atoms give N orbitals that
are closely spaced in energy.
N/2 are filled.
The valence band.
N/2 are empty.
The conduction band.
Metals - Properties
Crystal structure: Atoms
arranged in a regular
repeating structure
Relatively strong
Dense
Malleable and ductile: high
plasticity
Resistant to fracture: tough
Excellent conductors of
electricity and heat
Opaque to visible light
Shiny appearance
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Metal Alloys
Alloys are compounds consisting of more than one metal.
Adding other metals can affect the density, strength, fracture
toughness, plastic deformation, electrical conductivity and
environmental degradation.
Al-Fe = Iron will make it stronger.
Cu-Sn = bronze,
Cu-Zn = brass, Fe-C=steel,
Fe-C-Cr = Stainless steel
Pb-Sn = solder
Conductivity of Metals and Alloys
Mean free path - The average distance that electrons can
move without being scattered by other atoms.
Temperature Effect - When the temperature of a metal
increases, thermal energy causes the atoms to vibrate
Effect of Atomic Level Defects - Imperfections in crystal
structures scatter electrons, reducing the mobility and
conductivity of the metal
Matthiessen’s rule - The resistivity of a metallic material
is given by the addition of a base resistivity that
accounts for the effect of temperature (ρT), and a
temperature independent term that reflects the effect of
atomic level defects, including impurities forming solid
solutions (ρd). ρ= ρT + ρi + ρd
Effect of Processing and Strengthening
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Metals: Resistivity vs T, Impurities,defects
I) Imperfections increase resistivity
6
(10-8 Ohm-m)
Resistivity, ρ
--grain boundaries
--dislocations
--impurity atoms
--vacancies
5
4
3
2
1
0
These act to scatter
electrons so that they
take a less direct path.
Ni
at%
2
3
.
i
+3
i
Cu
t%N at%N
a
6
1
2
1.1
+ 2.
Cu
u+
C
ed
Ni
orm
f
at%
e
2
d
1
+ 1.
Cu
Cu
re”
“Pu
-200 -100
0
2) Resistivity
increases with:
--temperature
--wt% impurity
--%defect
ρ = ρ thermal
+ ρ imp
+ ρ def
T (°C)
from Fig. 18.8, Callister 6e
8
Metals: Resistivity vs T, Impurities,defects
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Movement of an electron through (a) a perfect crystal,
(b) a crystal heated to a high temperature, and
(c) a crystal containing atomic level defects.
Scattering of the electrons reduces the mobility and
conductivity.
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Estimating Conductivity
--Estimate the electrical conductivity of a Cu-Ni alloy
that has a yield strength of 125MPa.
180
160
140
120
100
21 wt%Ni
80
60
0 10 20 30 40 50
Resistivity, ρ
(10-8 Ohm-m)
Yield strength (MPa)
• Question:
wt. %Ni, (Concentration C)
Adapted from Fig.
7.14(b), Callister 6e.
50
40
30
20
10
0
0 10 20 30 40 50
wt. %Ni, (Concentration C)
ρ = 30x10 −8 Ohm − m
σ=
Adapted from Fig.
18.9, Callister 6e.
1
= 3.3x10 6 (Ohm − m) −1
ρ
9
Semi-conductors
Semiconductors are materials which have a conductivity between
conductors (generally metals) and nonconductors or insulators (such as
most ceramics).
Semiconductors have special electronic properties which allow them to be
insulating or conducting depending on their composition.
Semiconductors also have special optical properties when exposed to
electricity or light.
Several elements are semiconductors, the most important being silicon.
Semiconductors can be pure elements, such as silicon or germanium, or
compounds such as gallium arsenide or cadmium selenide.
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P-type
N-type
Semiconductor materials
Elemental semiconductors :
Group IV Elements
Silicon and Germanium
Compound semiconductors
III-V Compounds (e.g. GaAs, InSb)
II and VI Compounds (e.g. ZnS, CdSe)
III
IV
V
VI
Al
Si
P
S
Zn
Ga
Ge
As
Se
Cd
In
Sn
Sb
Te
II
Hg
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Properties of Semiconductors
Made primarily from metalloids
Regular arrangement of atoms (high quality crystals)
amorphous silicon, exception, used for thin film transistor and solar cells
Extremely controlled chemical purity
Adjustable conductivity of electricity
Opaque to visible light
Shiny appearance
Some have good plasticity, but others are fairly brittle.
Some have an electrical response to light.
Electrons In a Semiconductor
electrons
In semiconductors we are
pimarily interested in the
valence band and conduction
band.
A semiconductor has a
“small”
small” energy gap
For most applications we are
interested in what happens
near the top of the valence
band and the bottom of the
conduction band.
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Types of Semiconductors in Terms of Band Theory
Semiconductors:Holes
Holes are empty states in
the valence band created by
electrons that have jumped
to the conduction band
It is common to view the
conduction process in the
valence band as a flow of
positive holes toward the
negative electrode applied to
the semiconductor
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Current Process in Semiconductors
An external voltage is
supplied
Electrons move toward
the positive electrode
Holes move toward the
negative electrode
There is a symmetrical
current process in a
semiconductor
Doping in Semiconductors
Doping is the adding of impurities to a
semiconductor
Generally about
1 impurity atom per 107 semiconductor
atoms
Doping changes both the band structure and the
resistivity
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Intrinsic vs Extrinsic Conduction
• Intrinsic: # electrons = # holes (n = p)
example :pure Si
• Extrinsic: # electrons ≠ # holes (n ≠ p)
Example: when impurities are added with a different
# valence electrons than the host (e.g., Si atoms)
• N-type Extrinsic: (n >> p)
• P-type Extrinsic: (p >> n)
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Extrinsic Semiconductor alloys
Extrinsic Semiconductors:Electrical behaviour is
directed by the electronic structure of the
impurity element
Group V elements in Si:
P,As,Sb in Si (5 valence e)
Group III in Si:
Al,B,Ga in Si (3 valence e)
n type
semiconductor
p type
semiconductor
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Conduction in Terms of Electron and Hole Migration
• Concept of electrons and holes:
valence
electron
+
no applied
electric field
electron hole
pair migration
electron hole
pair creation
Si atom
- +
applied
electric field
applied
electric field
Adapted from Fig. 18.10,
Callister 6e.
11
n-type Semiconductors
Negative electrons are charge carriers and so called n-type.
Donor atoms are doping
materials that contain one
more electron than the
semiconductor material
This creates an essentially
free electron with an
energy level in the energy
gap, just below the
conduction band
Only a small amount of
thermal energy is needed
to cause this electron to
move into the conduction
band
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p-type Semiconductors
There is a hole in the valence band to accept electron and so called p-type.
Electron Acceptor atoms are doping
materials that contain one less electron
than the semiconductor material
A hole is left where the missing
electron would be
The energy level of the hole lies in the
energy gap, just above the valence
band
An electron from the valence band has
enough thermal energy to fill this
impurity level, leaving behind a hole in
the valence band
Pure Semiconductors: Conductivity vs T
• Data for Pure Silicon:
--σ increases with T
--opposite to metals
electrical conductivity,σ
(Ohm-m)-1
102
101
100
10-1
pure
(undoped)
10-2
50 100
Fig. 19.15, Callister 5e.
1000
T(K)
−Egap / kT
Energy
empty
band
?
GAP
filled states
104
103
σundoped ∝ e
filled
valence
band
electrons
can cross
gap at
higher T
filled
band
material
Si
Ge
GaP
CdS
band gap (eV)
1.11
0.67
2.25
2.40
Table 18.2, Callister 6e.
10
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Electrical Conduction vs Temperature
1)Thermal energy puts many electrons into a
higher energy state.
Click to animate
2)Photon effect on conductivity
Click to animate
Conductivity vs T: Doped Semiconductors
0.0052at%B
102
101
doped
0.0013at%B
100
10-1
pure
(undoped)
10-2
50 100
1000
T(K)
Adapted from Fig. 19.15, Callister 5e.
1021/m3 of a n-type donor
impurity (such as P).
--for T < 100K: "freeze-out"
thermal energy insufficient to
excite electrons.
--for 150K < T < 450K: "extrinsic"
--for T >> 450K: "intrinsic"
doped
undoped
3
2
1
Adapted from Fig.
18.16, Callister 6e.
intrinsic
104
103
extrinsic conduction...
--extrinsic doping level:
extrinsic
electrical conductivity, σ
(Ohm-m)-1
lower the activation energy to
produce mobile electrons.
• Comparison: intrinsic vs
freeze-out
--σ increases with doping
--reason: imperfection sites
conduction electron
concentration (1021/m3)
• Data for Doped Silicon:
0
0
200 400 600 T(K)
13
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Insulators
The valence band is
completely full of
electrons
A large band gap
separates the valence and
conduction bands
A large amount of energy
is needed for an electron
to be able to jump from
the valence to the
conduction band
The minimum required
energy is Eg
Energy
empty
band
GAP
filled states
filled
valence
band
filled
band
Semiconductors: summary of def’ns
Intrinsic semiconductor - A semiconductor in which
properties are controlled by the element or compound that
makes the semiconductor and not by dopants or impurities.
Extrinsic semiconductor - A semiconductor prepared by
adding dopants, which determine the number and type of
charge carriers.
Doping - Deliberate addition of controlled amounts of other
elements to increase the number of charge carriers in a
semiconductor.
Thermistor - A semiconductor device that is particularly
sensitive to changes in temperature, permitting it to serve
as an accurate measure of temperature.
Radiative recombination - Recombination of holes and
electrons that leads to emission of light; this occurs in
direct bandgap materials.
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Applications of Semiconductors
Diodes, transistors, lasers, and LEDs are made using
semiconductors. Silicon is the workhorse of very large scale
integrated (VLSI) circuits.
Forward bias - Connecting a p-n junction device so that the pside is connected to positive. Enhanced diffusion occurs as the
energy barrier is lowered, permitting a considerable amount
of current can flow under forward bias(bias :meyil/eğilim).
Reverse bias - Connecting a junction device so that the p-side
is connected to a negative terminal; very little current flows
through a p-n junction under reverse bias.
Avalanche breakdown - The reverse-bias voltage that causes
a large current flow in a lightly doped p-n junction.
Transistor - A semiconductor device that can be used to
amplify electrical signals.
Semiconductor Devices
Advantages of semiconductor devices( solid-state
devices):
1. Small size
2. Low power consumption
3. No warmup time
p-n Rectifying junction:
A rectifier or diode is an electronic device that
allows the current to flow in one direction only
A diode transforms an alternating current into
direct current
Rectifying: düzeltmek, alternatif akımı doğru akıma çevirmek
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p-n Rectifying Junction
• Processing: A single piece of semiconductoris doped so
as to be n-type on one side and p-type on the other.
Diffuse P(Gr VA) into one side of a B-doped (Gr.IIIA)
crystal.
p-type
n-type
+
+ +
+ +
1)No applied potential:
no net current flow.
2)Forward bias: carrier
flow through p-type and
n-type regions; holes and
electrons recombine at
p-n junction; current flows.
3)Reverse bias: carrier
flow away from p-n junction;
carrier conc. greatly reduced
at junction; little current flow.
p-type
+
-
+
-
-
-
-
-
+ - n-type
+
+ - - +-
+p-type
+ +
+ +
n-type
-
-
-
+
14
Behavior of a p-n junction
device: (a) When no bias is
applied electron and hole
currents due to drift and
diffusion cancel out and
there is no net current
(b) Under a forward bias
causes the potential barrier
to be reduced causing a
current to flow.
(c) Under reverse bias, the
potential barrier increases
and very little current flows.
(Source: From Solid State
Electronic Devices, Third
Edition, by. B.G. Streetman,
Fig. 5-10. Copyright © 2000
Prentice Hall. Reprinted by
permission Pearson Education,
Inc., Upper Saddle River, NJ.)
Drift: yığılmak, birikmek, sürüklenmek.
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Applications of Semiconductors
Semiconductors can be made into
transistors which can store a digital (on or
off) signal. Transistors are used in all
electronics--computers, calculators, stop
watches, amplifiers, etc.
Semiconductors can also be made to emit
light by exposing them to an electric field
(light emitting diodes (LEDs) and diode
lasers).
Semiconductors are used in computers
(45%), consumer products (23%),
communications equipment (13%),
manufacturing industries (12%),
automobiles (5%), and by the military
(2%).
Semiconductors make up a huge, and
growing, industry worldwide with billions
of dollars in sales by semiconductor
companies in North America, Europe, and
the Asia-Pacific region.
Advances in the field of electronics can
continue to improve our lives.
Photovoltaic Cells
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Moore's Law (1965 then 1975 )
the number of transistors on a chip doubles about
every two years.
Moore's Law (1975)
In 1965, a single transistor cost more than a dollar.
By 1975, the cost of a transistor had dropped to less than a
penny, while transistor size allowed for almost 100,000
transistors on a single die.
From 1979 to 1989, to 1999, processor performance went from
about 1.5 million instructions per second (MIPS), to almost 50
MIPS on the i486™, to over 1,000 MIPS on the Intel®
Pentium® III.
Today's Intel® processors, some topping out at well above 1
billion transistors, run at 3.2 GHz and higher, deliver over
10,000 MIPS, and can be manufactured in high volumes with
transistors that cost less than 1/10,000th of a cent.
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Moore's Law (1975)
Teramac Computer,1999
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A current bit on a hard drive is 300 nm by
100 nm
The short history of Silicon Transistors:
In 1955, Texas Instruments work to develop
silicon transistors for military use.
It was recognized that the germanium transistors
then available would not be able to perform at the
high temperatures common to military
applications.
TI had invented the silicon transistor, the first of
its kind.
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What is a transistor?
CPU (Central Processing Unit)
The "brain" of the computer that performs most computing
tasks. In microcomputers, the entire CPU is on a single chip.
Also called a processor/microprocessor.
The on and off switches found inside every single piece of computer equipment. In a
computer, this on/off ability is used to represent binary digits (bits). A CPU contains
millions of transistors. www.pcstats.com/glossary.cfm
An active solid-state device in which larger output current is obtained by small
changes in the input current.www.sfxaudio.com/AudioSchool/glossary.asp
Transistors are tiny electrical devices that can be found in everything from radios to
robots. They have two key properties: 1) they can amplify an electrical signal and 2)
they can switch on and off, letting current through or blocking it as necessary.
www.pbs.org/transistor/glossary.html
A device used to amplify a signal or open and close a circuit. In a computer, it
functions as an electronic switch.www.synopsys.com/news/pr_kit/eda_glossary.html
An electronic device that can regulate electricity and act as an on/off switch.
www.pccomputernotes.com/pcterms/glossaryt.htm
An active component of an electronic circuit consisting of a small block of
semiconducting material. It may be used as an amplifier, detector, or switch.
www.csa.com/hottopics/mems/gloss.php
A small chip of semiconductor material that amplifies or switches electrical current.
www.southerncompany.com/learningpower/glossary.asp
Transistors made from purified silicon. There are also polymer transistors.
www.rolltronics.com/links/glossary.html
View of an Integrated Circuit
• Scanning electron microscope images of an IC:
Al
Si
(doped)
(d)
(a)
45µm
0.5mm
• A dot map showing location of Si (a semiconductor):
--Si shows up as light regions.
(b)
• A dot map showing location of Al (a conductor):
--Al shows up as light regions.
Fig. (d) from Fig. 18.25, Callister 6e. (Fig. 18.25 is
courtesy Nick Gonzales, National Semiconductor
Corp., West Jordan, UT.)
(c)
Fig. (a), (b), (c) from Fig. 18.0,
Callister 6e.
2
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Microelectronic Circuit
SEM of a small region of a microprocessing chip( 0.5
MB)Narrow white regions Al wiring,the gray regions
doped Si ( x2000)
Optical Photomicrograph of a part of a circuit to test
microprocessing chips( 0.5 MB)Narrow light regions Al
wiring,the white squares are test semiconductor devices ( x50)
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The Transistor
Capable of two primary types of function
can amplify an electrical signal
serve as switching devices in computers for the
processing and storage of information
•Two major types
1)The junction(bipolar)transistor
2)Metal-Oxide-Semiconductor Field-Effect
Transistor ( MOSFET)
The Junction(bipolar)transistor
Two p-n junctions arranged back to back
oA very thin n type base is sandwiched in
between p type emitter and p type
collector.
oEmitter p is forward bias, so that large
number of holes enter n-type base region.
oThese holes are minorty type carriers in
n-type base,some will combine with
electrons.
oMost of them will be pass across
junction 2 to collector.
oThe holes become a part of the emittercollector circuit.
oA small increase in input voltage
produces a large increase in output
voltage.
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The Junction(bipolar)transistor
Bipolar transistors find important uses as
amplifiers,
drivers of other devices, and
certain high speed digital circuits.
Metal-Oxide-Semiconductor Field-Effect Transistor
( MOSFET)
oTwo
Two small islands
of pp-type
semiconductors in
n-type silicon.
silicon.
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Insulators and Dielectric Properties
Materials used to insulate an electric field from its
surroundings are required in a large number of electrical
and electronic applications.
Electrical insulators obviously must have a very low
conductivity, or high resistivity, to prevent the flow of
current.
Porcelain, alumina, mica, and some glasses and plastics
are used as insulators.
Dielectric Materials,Polarization
A dielectric material is electrically
insulating and may exhibit a
dipole structure.
Dielectric materials are used in
capacitors and insulators.
eg.glass,porcelain,mica,
TiO2 and BaTiO3
Polarization:The process of dipole
alignment.
Types: Electronic,ionic,orientation
Figure shows an electric dipole generated by
two electric charges separated by distance d
P is polarization vector
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Polarization in Dielectrics
Capacitor - A microelectronic device, constructed from
alternating layers of a dielectric and a conductor, that is
capable of storing a charge.These can be single layer or multilayer devices.
Dielectric strength - The maximum electric field that can be
maintained between two conductor plates without causing a
breakdown.
(a) a charge can be stored at the conductor plates in a vacuum
(b) when a dielectric is placed between the plates the dielectric
polarizes and additional charge is stored.
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Ferroelectric Materials,Piezoelectric Materials
Ferroelectrics are dielectric material
which exhibit continuous polarity even in
the absence of external electric field.
Have extremely high dielectric costants
Ferroelectric capacitors are smaller than
the other capacitors.
eg. Pb(ZrO3 )(TiO3), Bismuth tungstate
Piezoelectricity is the phenomenon where
polarization is induced in a material by the
application of external forces.
Piezoelectrics - Materials that develop
voltage upon the application of a stress and
develop strain when an electric field is
applied.
Application: pickups, microphones, sonar
detectors etc.
PbZrO3 , quartz etc.
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
The (a) direct and (b) converse piezoelectric effect. In the
direct piezoelectric effect, applied stress causes a voltage to
appear. In the converse effect (b), an applied voltage leads
to development of strain.
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