Chapter 18 - 1 ISSUES TO ADDRESS... • How are electrical conductance and resistance characterized? • What are the physical phenomena that distinguish conductors, semiconductors, and insulators? • For metals, how is conductivity affected by imperfections, temperature, and deformation? • For semiconductors, how is conductivity affected by impurities (doping) and temperature? Chapter 18: Electrical Properties Chapter 18 - 2 • Scanning electron micrographs of an IC: Fig. (d) from Fig. 12.27(a), Callister & Rethwisch 3e. (Fig. 12.27 is courtesy Nick Gonzales, National Semiconductor Corp., West Jordan, UT.) • A dot map showing location of Si (a semiconductor): -- Si shows up as light regions. (b) View of an Integrated Circuit 0.5mm (a) (d) 45 m Al Si (doped) (d) • A dot map showing location of Al (a conductor): -- Al shows up as light regions. (c) Figs. (a), (b), (c) from Fig. 18.27, Callister & Rethwisch 8e.
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Chapter 18 - 1
ISSUES TO ADDRESS...
• How are electrical conductance and resistancecharacterized?
• What are the physical phenomena that distinguishconductors, semiconductors, and insulators?
• For metals, how is conductivity affected byimperfections, temperature, and deformation?
• For semiconductors, how is conductivity affectedby impurities (doping) and temperature?
Chapter 18: Electrical Properties
Chapter 18 - 2
• Scanning electron micrographs of an IC:
Fig. (d) from Fig. 12.27(a), Callister & Rethwisch 3e.(Fig. 12.27 is courtesy Nick Gonzales, National Semiconductor Corp., West Jordan, UT.)
• A dot map showing location of Si (a semiconductor):-- Si shows up as light regions.
(b)
View of an Integrated Circuit
0.5mm
(a)(d)
45m
Al
Si (doped)
(d)
• A dot map showing location of Al (a conductor):-- Al shows up as light regions. (c)
These act to scatterelectrons so that theytake a less direct path.
• Resistivityincreases with:
=
Adapted from Fig. 18.8, Callister & Rethwisch 8e. (Fig. 18.8 adapted from J.O. Linde, Ann. Physik 5, p. 219 (1932); and C.A. Wert and R.M. Thomson, Physics of Solids, 2nd ed., McGraw-Hill Book Company, New York, 1970.)
T (ºC)-200 -100 0
1
2
3
4
5
6
Res
istiv
ity,
(10
-8O
hm-m
)
0
d-- %CW
+ deformation
i
-- wt% impurity
+ impurity
t
-- temperature
thermal
Chapter 18 -13
Estimating Conductivity
Adapted from Fig. 7.16(b), Callister & Rethwisch 8e.
• Question:-- Estimate the electrical conductivity of a Cu-Ni alloy
that has a yield strength of 125 MPa.
mOhm10 x 30 8
16 )mOhm(10 x 3.31
Yie
ld s
tren
gth
(MP
a)
wt% Ni, (Concentration C)0 10 20 30 40 50
6080
100120140160180
21 wt% Ni
Adapted from Fig. 18.9, Callister & Rethwisch 8e.
wt% Ni, (Concentration C)R
esis
tivity
,
(10
-8O
hm-m
)
10 20 30 40 500
10203040
50
0
125
CNi = 21 wt% Ni
From step 1:
30
Chapter 18 -14
Charge Carriers in Insulators and Semiconductors
Two types of electronic charge carriers:
Free Electron – negative charge – in conduction band
Hole– positive charge– vacant electron state in
the valence band
Adapted from Fig. 18.6(b), Callister & Rethwisch 8e.
Move at different speeds - drift velocities
Chapter 18 -15
Intrinsic Semiconductors
• Pure material semiconductors: e.g., silicon & germanium
– Group IVA materials
• Compound semiconductors
– III-V compounds• Ex: GaAs & InSb
– II-VI compounds• Ex: CdS & ZnTe
– The wider the electronegativity difference between the elements the wider the energy gap.
Chapter 18 -16
Intrinsic Semiconduction in Terms of Electron and Hole Migration
Adapted from Fig. 18.11, Callister & Rethwisch 8e.
electric field electric field electric field
• Electrical Conductivity given by:
# electrons/m3 electron mobility
# holes/m3
hole mobilityhe epen
• Concept of electrons and holes:
+-
electron holepair creation
+-
no applied applied
valence electron Si atom
applied
electron holepair migration
Chapter 18 -17
Number of Charge Carriers
Intrinsic Conductivity
)s/Vm 45.085.0)(C10x6.1(
m)(10219
16
hei e
n
For GaAs ni = 4.8 x 1024 m-3
For Si ni = 1.3 x 1016 m-3
• Ex: GaAs
he epen
• for intrinsic semiconductor n = p = ni
= ni|e|(e + h)
Chapter 18 -18
Intrinsic Semiconductors: Conductivity vs T
• Data for Pure Silicon:-- increases with T-- opposite to metals
Adapted from Fig. 18.16, Callister & Rethwisch 8e.
materialSiGeGaPCdS
band gap (eV)1.110.672.252.40
Selected values from Table 18.3, Callister & Rethwisch 8e.
ni eEgap / kT
ni e e h
Chapter 18 -19
• Intrinsic:-- case for pure Si-- # electrons = # holes (n = p)
• Extrinsic:-- electrical behavior is determined by presence of impurities
that introduce excess electrons or holes-- n ≠ p
Intrinsic vs Extrinsic Conduction
3+
• p-type Extrinsic: (p >> n)
no applied electric field
Boron atom
4+ 4+ 4+ 4+
4+
4+4+4+4+
4+ 4+ hep
hole
• n-type Extrinsic: (n >> p)
no applied electric field
5+
4+ 4+ 4+ 4+
4+
4+4+4+4+
4+ 4+
Phosphorus atom
valence electron
Si atom
conductionelectron
een
Adapted from Figs. 18.12(a) & 18.14(a), Callister & Rethwisch 8e.
Chapter 18 -20
Extrinsic Semiconductors: Conductivity vs. Temperature
• Data for Doped Silicon:-- increases doping-- reason: imperfection sites
lower the activation energy toproduce mobile electrons.
• Comparison: intrinsic vsextrinsic conduction...
-- extrinsic doping level:1021/m3 of a n-type donorimpurity (such as P).
-- for T < 100 K: "freeze-out“,thermal energy insufficient toexcite electrons.
-- for 150 K < T < 450 K: "extrinsic"-- for T >> 450 K: "intrinsic"
Adapted from Fig. 18.17, Callister & Rethwisch 8e. (Fig. 18.17 from S.M. Sze, Semiconductor Devices, Physics, and Technology, Bell Telephone Laboratories, Inc., 1985.)
Con
duct
ion
elec
tron
conc
entr
atio
n (1
021 /
m3 )
T (K)6004002000
0
1
2
3
free
ze-o
ut
extr
insi
c
intr
insi
c
doped
undoped
Chapter 18 -21
• Allows flow of electrons in one direction only (e.g., usefulto convert alternating current to direct current).
• Processing: diffuse P into one side of a B-doped crystal.
-- No applied potential:no net current flow.
-- Forward bias: carriersflow through p-type andn-type regions; holes andelectrons recombine atp-n junction; current flows.
-- Reverse bias: carriersflow away from p-n junction;junction region depleted of carriers; little current flow.
p-n Rectifying Junction
++
++
+- ---
-p-type n-type
+ -
++ +
++
--
--
-
p-type n-typeAdapted from Fig. 18.21 Callister & Rethwisch 8e.
• Integrated circuits - state of the art ca. 50 nm line width
– ~ 1,000,000,000 components on chip
– chips formed one layer at a time
Fig. 18.26, Callister & Rethwisch 8e.
• MOSFET (metal oxide semiconductor field effect transistor)
Chapter 18 -
Capacitance (C)
25
Chapter 18 -
26
Chapter 18 -
Electric Dipole Moment (p)
27
Chapter 18 -
Surface charge density (D)
28
Chapter 18 -
Effect of Polarization
29
Chapter 18 -30
Chapter 18 -
Example
31
Chapter 18 -32
Chapter 18 -
Types of Polarization
33
Chapter 18 -
Frequency dependence of Dielectric Constant
34
Chapter 18 -35
Ferroelectric Ceramics
• Experience spontaneous polarization
Fig. 18.35, Callister & Rethwisch 8e.
BaTiO3 -- ferroelectric below its Curie temperature (120ºC)
Chapter 18 -36
Piezoelectric Materials
stress-free with applied stress
Adapted from Fig. 18.36, Callister & Rethwisch 8e. (Fig. 18.36 from Van Vlack, Lawrence H., Elements of Materials Science and Engineering, 1989, p.482, Adapted by permission of Pearson Education, Inc., Upper Saddle River, New Jersey.)
Piezoelectricity– application of stress induces voltage– application of voltage induces dimensional change
Chapter 18 -37
• Electrical conductivity and resistivity are:-- material parameters-- geometry independent
• Conductors, semiconductors, and insulators...-- differ in range of conductivity values-- differ in availability of electron excitation states
• For metals, resistivity is increased by-- increasing temperature-- addition of imperfections-- plastic deformation
• For pure semiconductors, conductivity is increased by-- increasing temperature-- doping [e.g., adding B to Si (p-type) or P to Si (n-type)]
• Other electrical characteristics-- ferroelectricity-- piezoelectricity
• When material is under stress, it generates Dipole in the material.• Amount of Dipole is linearly proportional of applied stress.• On the other hand, when applying voltage, it generate stress in the material, and may cause change of shape of material. • Application: ultrasonic device, microphone, phonograph cartridge, accelerometer, strain gauges, sonar devices, actuator
Chapter 18 -
• With appropriate applied force in a direction, it will generate Dipole in the unit cell of Piezoelectric ceramic, causing voltage in the material.• On the other hand, applied voltage causing distortion of unit cell.
Chapter 18 -
Chapter 18 -
Fig. Edison cylinder phonograph: 1899
Source:http://en.wikipedia.org/wiki/Phonograph
Air pressure from wave of sound is store as roughness on the surface of the wax.
Roughness causes vibration at stylus (pen) and causes stress to the piezoelectric ceramic.
Chapter 18 -
Voltage signal from amplifier cause contraction and expansion of the piezoelectric ceramic.
That causes vibration of the diaphragm of the speaker of the headphone.
Chapter 18 -
Hydrophone is a device for listening to sound under water.
Accelerometer is a device for measuring mass movement or acceleration.
Chapter 18 -
Chapter 18 -
Fig. 7.38: Piezoelectric transducers are widely used to generateultrasonic waves in solids and also to detect such mechanicalwaves. The transducer on the left is excited from an ac sourceand vibrates mechanically. These vibrations are coupled to thesolid and generate elastic waves. When the waves reach theother end they mechanically vibrate the transducer on the rightwhich converts the vibrations to an electrical signal.
Configuration is similar to a capacitor. The input is voltage, the output is small movement due to distortion of crystal.
Chapter 18 -
Piezoelectric Multilayer Bender Actuator• Positioning Range up to 2 mm • Fast Response ( 10 msec) • Nanometer Scale Resolution • Low Operating Voltage (0 to 60 V) • Low Temperature Compatible
Chapter 18 -
A piezoelectric nozzle [above] works when a current bends a piezoelectric crystal, forcing the fluid down and out of the nozzle. To heat and expand the fluid, instead of a crystal. Piezoelectric nozzles create very smaller droplets.