© 2008 Brooks/Cole
1
Chapter 20, section 20.3
Semiconductors and superconductors
© 2008 Brooks/Cole
2
Metallic Bonds: The “electron” sea model
Valence electrons can move anywhere within the metal
These electrons are delocalized (similar to resonance)
© 2008 Brooks/Cole
3
To understand metallic bonding and electrical conductivity,
we need to re
-
visit molecular orbital theory.
© 2008 Brooks/Cole
4
1
Number of
atoms
e
-
in
atoms
e
-
in lattice
energy levels
3
2
12
many
filled
empty
Formation of bands of valence orbitals in alkali metal
A metal behaves like a giant molecule
Valence orbitals extend
over the entire metal
Electrons in partially filled valence band
are free to move throughout the metal
© 2008 Brooks/Cole
5
What if the valence band is full as in Mg?
Electrons can be excited to empty “conduction band”!
© 2008 Brooks/Cole
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So why don’t all solids conduct electricity?
Metals, Semiconductors & Insulators
Silicon and germanium form network solids similar to diamond
but they are semiconductors.
© 2008 Brooks/Cole
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Doped semiconductors: n
-
type and p
-
type
Si doped with As or P
Extra electron is all by itself
in the conduction band!
n
-
type semiconductor
Si doped with B
“hole” (positive charge)
conducts electricity!
p
-
type semiconductor
© 2008 Brooks/Cole
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Movement of holes requires concerted
movement of large numbers of electrons
Hole (+)
Electron (
-
)
Equivalent to
Which type of semiconductor
is a better conductor? Why?
Analogy courtesy of Dr. Joe Lomax
N
-
type.
Holes are less mobile than electrons
© 2008 Brooks/Cole
9
Current flows in only one direction through p
-
n junction!
A
p
-
n
junction (rectifier)
.
Charge carriers: holes (
p
-
type) and e
-
(
n
-
type)
© 2008 Brooks/Cole
10
Solar panels employ a p
-
n junction
A
photovoltaic cell
. Light drives e
-
around an external circuit
e
-
© 2008 Brooks/Cole
11
Metals are better conductors at low
T.
Superconductors have zero resistance (are perfect
conductors) at some (low)
T.
Superconductors
Resistance
Temperature (K)
T
c
critical
T
0
Wrong! Should approach
zero resistance!
Why are super
conductors useful?
© 2008 Brooks/Cole
12
Metal
T
c
(K)
Aluminum
1.15
Gallium
1.10
Tin
3.72
Mercury
4.15
Lanthanum
4.9
Lead
7.2
Several metals are superconductors at low
T
Nb
3
Sn alloy
18.1
YBa
2
Cu
3
O
7
90.
LaBa
2
Cu
3
O
x
35.
He(
l
) boils at 4.2 K
Many He(
l
)
-
cooled
magnets use Nb
-
alloys
1
st
ceramic
superconductor (1986).
N
2
(
l
) boils at 77 K
“Y123” found 4 months later.
Highest
T
c
so far…
Hg
0.8
T
l
0.2
Ba
2
Ca
2
Cu
3
O
8.23
138.
Some alloys are better…
Superconductors
© 2008 Brooks/Cole
13
Levitating train can travel at >300 mph!
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