Semiconductors and superconductors

© 2008 Brooks/Cole

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Chapter 20, section 20.3

Semiconductors and superconductors

© 2008 Brooks/Cole

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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.

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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

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What if the valence band is full as in Mg?

Electrons can be excited to empty “conduction band”!


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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.

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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

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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

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Current flows in only one direction through p
-
n junction!

A
p
-
n
junction (rectifier)
.

Charge carriers: holes (
p
-
type) and e
-

(
n
-
type)

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Solar panels employ a p
-
n junction

A
photovoltaic cell
. Light drives e
-

around an external circuit

e
-

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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

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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

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Levitating train can travel at >300 mph!