Transcript Electronic Materials

Electronic Materials
An Overview
R. Lindeke
View
of
an
Integrated
Circuit
• Scanning electron microscope images of an IC:
Al
Si
(doped)
(d)
(d)
(a)
45 mm
0.5 mm
• 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.
!courtesy Nick Gonzales, National Semiconductor Corp.,
West Jordan, UT.)
(c)
Fig. (a), (b), (c) from Fig. 18.0,
Callister 7e.
2
Electrical
Conduction
• Ohm's Law:
V = I R
voltage drop (volts = J/C)
resistance (Ohms)
current (amps = C/s)
C = Coulomb
A
e-
(cross
sect.
area)
• Resistivity, r and Conductivity, s:
I
V
L
-- geometry-independent forms of Ohm's Law
-- Resistivity is a material property & is independent of sample
E: electric
field
intensity
• Resistance:
V
I
 r
L
A
rL
L
R

A As
resistivity
(Ohm-m)
J: current density
conductivity
1
s
r
3
Electrical Properties
• Which will conduct more electricity?
D
2D
RA
VA
r


I
• Analogous to flow of water in a pipe
• So resistance depends on sample
geometry, etc.
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Definitions
Further definitions
J = s  <= another way to state Ohm’s law
J  current density
current
I


surface area A
like a flux
  electric field potential = V/ or (V/ )
J = s (V/ )
Electron flux
conductivity
voltage gradient
Current carriers
• electrons in most solids
• ions can also carry (particularly in liquid solutions)
5
Conductivity:
Comparison
-1 = ( - m) -1
• Room T values (Ohm-m)
METALS
conductors
CERAMICS
Silver
6.8 x 10
7
Soda-lime glass 10
Copper
6.0 x 10
7
Concrete
Iron
1.0 x 10
7
Aluminum oxide
SEMICONDUCTORS
10
-9
<10
-13
<10
-14
POLYMERS
Silicon
4 x 10 -4
Germanium
2 x 10
GaAs
-10-10 -11
0
Polystyrene
Polyethylene
10
-15-10 -17
10 -6
insulators
semiconductors
Selected values
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Example: Conductivity Problem
What is the minimum diameter (D) of the wire so that V < 1.5 V?
e-
Cu wire -
100m
I = 2.5A
+
V
100m
D 2
4
Solve to get
L
V
R

As
I
< 1.5V
2.5A
6.07 x 107 (Ohm-m)-1
D > 1.87 mm
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Electronic Band Structures
Adapted from Fig. 18.2, Callister 7e.
8
Band Structure
• Valence band – filled – highest occupied energy levels
• Conduction band – empty – lowest unoccupied energy levels
Conduction
band
valence band
Adapted from Fig. 18.3, Callister 7e.
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Conduction & Electron Transport
• Metals (Conductors):
-- Thermal energy puts
many electrons into
a higher energy state.
-
Energy
Energy
empty
band
empty
band
GAP
partly
filled
valence
band
filled
band
filled states
-- for metals nearby
energy states
are accessible
by thermal
fluctuations.
+
filled states
• Energy States:
-
filled
valence
band
filled
band
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Energy States: Insulators &
Semiconductors
• Insulators:
• Semiconductors:
-- Higher energy states not
-- Higher energy states separated
accessible due to gap (> 2 eV). by smaller gap (< 2 eV).
Energy
Energy
empty
band
filled
valence
band
filled
band
?
GAP
filled states
filled states
GAP
empty
band
filled
valence
band
filled
band
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Charge Carriers
Adapted from Fig. 18.6 (b), Callister 7e.
Two charge carrying mechanisms
Electron – negative charge
Hole
– equal & opposite
positive charge
Move at different speeds - drift
velocity
Higher temp. promotes more electrons into the conduction band

s as T
Electrons scattered by impurities, grain boundaries, etc.
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Metals: Resistivity vs T, Impurities
• Imperfections increase resistivity
-- grain boundaries
-- dislocations
-- impurity atoms
-- vacancies
These act to scatter
electrons so that they
take a less direct path.
(10 -8 Ohm-m)
Resistivity,
r
6
• Resistivity
5
increases with:
4
-- temperature
-- wt% impurity
-- %CW
3
2
1
0
-200
-100
0
T (°C)
(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.)
r = rthermal
+ rimpurity
+ rdeformation
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Estimating
Conductivity
• Question:
180
160
140
125
120
100
21 wt%Ni
80
60
0 10 20 30 40 50
Resistivity, r
(10 -8 Ohm-m)
Yield strength (MPa)
-- Estimate the electrical conductivity s of a Cu-Ni alloy
that has a yield strength of 125 MPa.
wt. %Ni, (Concentration C)
From step 1:
CNi = 21 wt%Ni
Adapted from Fig.
18.9, Callister 7e.
50
40
30
20
10
0
0 10 20 30 40 50
wt. %Ni, (Concentration C)
r  30x108 Ohm  m
1
s   3.3x106 (Ohm  m)1
r
14
-- s increases with T
-- opposite to metals
electrical conductivity, s
(Ohm-m) -1
10 4
10 3
10 2
10 1
10 0
10 -1
10 -2
pure
(undoped)
50 10 0
sundoped  e
1000
T(K)
(from G.L. Pearson and J. Bardeen, Phys. Rev. 75, p.
865, 1949.)
 Egap / kT
Energy
empty
band
?
GAP
filled states
•
Pure Semiconductors:
Conductivity vs T
Data for Pure Silicon:
electrons
filled
can cross
valence gap at
band
higher T
filled
band
material
Si
Ge
GaP
CdS
band gap (eV)
1.11
0.67
2.25
2.40
Selected values.
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Conduction in Terms of Electron and Hole
Migration
• Concept of electrons and holes:
valence
electron
hole
Si
atom
electron
pair creation
+ -
-
no applied
electric field
electron
hole
pair migration
applied
electric field
+
applied
electric field
• Electrical Conductivity given by:
# holes/m 3
s  n e me  p e m h
# electrons/m3
hole mobility
electron mobility
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• Intrinsic:
Intrinsic vs Extrinsic Conduction
# electrons = # holes (n = p)
--case for pure Si
• Extrinsic:
--n ≠ p
--occurs 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)
Phosphorus atom
4+ 4+ 4+ 4+
s  n e me
4+ 5+ 4+ 4+
4+ 4+ 4+ 4+
no applied
electric field
Boron atom
hole
conduction
electron
4+ 4+ 4+ 4+
valence
electron
4+ 4+ 4+ 4+
Si atom
4+ 3+ 4+ 4+
no applied
electric field
s  p e mh
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p-n
Rectifying
Junction
• Allows flow of electrons in one direction only (e.g., useful
to convert alternating current to direct current.
• Processing: diffuse P into one side of a B-doped crystal.
• Results:
n-type
+ p-type
+
--No applied potential:
no net current flow.
--Forward bias: carrier
flow through p-type and
n-type regions; holes and
electrons recombine at
p-n junction; current flows.
--Reverse bias: carrier
flow away from p-n junction;
carrier conc. greatly reduced
at junction; little current flow.
+
+
-
-
+
+
+ + +
+ -
+ p-type
+
+
+
+
-
-
+
p-type
-
-
n-type
-
-
n-type
-
-
-
+
-
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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.
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Doped Semiconductor: Conductivity vs. T
10 4
0.0052at%B
10 3
10 2
doped
0.0013at%B
-- extrinsic doping level:
1021/m3 of a n-type donor
impurity (such as P).
-- for T < 100 K: "freeze-out“,
thermal energy insufficient to
excite electrons.
-- for 150 K < T < 450 K: "extrinsic"
-- for T >> 450 K: "intrinsic"
10 1
10 -1
pure
(undoped)
10 -2
50 100
1000
T(K)
(from G.L. Pearson and J. Bardeen, Phys. Rev. 75, p.
865, 1949.)
doped
undoped
3
freeze-out
10 0
conduction electron
concentration (1021/m3)
electrical conductivity, s
(Ohm-m) -1
lower the activation energy to
produce mobile electrons.
intrinsic vs
extrinsic conduction...
2
1
intrinsic
-- s increases doping
-- reason: imperfection sites
• Comparison:
extrinsic
• Data for Doped Silicon:
(S.M. Sze,
Semiconductor
Devices, Physics, and
Technology, Bell
Telephone
Laboratories, Inc.,
1985.)
0
0
200 400 600 T(K)
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
Number of Charge Carriers
Intrinsic Conductivity
s = n|e|me + p|e|me
• for intrinsic semiconductor n = p

s = n|e|(me + mn)
• Ex: GaAs
s
106 ( m)1
n

e me  mn
(1.6x1019 C)(0.85  0.45 m2/V  s)


For GaAs
n = 4.8 x 1024 m-3
For Si n = 1.3 x 1016 m-3
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Properties of Rectifying Junction
22
Transistor MOSFET
• MOSFET (metal oxide semiconductor field effect
transistor)
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Integrated Circuit Devices
• Integrated circuits - state of the art ca. 50 nm line width
– 1 Mbyte cache on board
– > 100,000,000 components on chip
– chip formed layer by layer
• Al is the “wire”
24
Ferroelectric Ceramics
Ferroelectric Ceramics are dipolar below Curie TC = 120ºC
• cooled below Tc in strong electric field - make material with
strong dipole moment
Fig. 18.35, Callister 7e.
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Piezoelectric Materials
Piezoelectricity – application of pressure produces current
at
rest
compression
induces
voltage
applied voltage
induces
expansion
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Summary
• Electrical conductivity and resistivity are:
-- material parameters.
-- geometry independent.
• Electrical resistance is:
-- a geometry and material dependent parameter.
• Conductors, semiconductors, and insulators...
-- differ in accessibility of energy states for
conductance electrons.
• For metals, conductivity is increased by
-- reducing deformation
-- reducing imperfections
-- decreasing temperature.
• For pure semiconductors, conductivity is increased by
-- increasing temperature
-- doping (e.g., adding B to Si (p-type) or P to Si (n-type).
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