© 2008 Brooks/Cole 1

Chapter 20, section 20.3Semiconductors and superconductors

© 2008 Brooks/Cole 2

Metallic Bonds: The “electron” sea modelMetallic Bonds: The “electron” sea model

Valence electrons can move anywhere within the metalThese 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 ofatoms

e- inatoms

e- in latticeenergy 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 6

So why don’t all solids conduct electricity?Metals, Semiconductors & Insulators

Silicon and germanium form network solids similar to diamondbut they are semiconductors.

© 2008 Brooks/Cole 7

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 8

Movement of holes requires concerted movement of large numbers of electrons

Hole (+)

Electron (-)

Equivalent to

Which type of semiconductoris 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 A p-n p-n junction (rectifier)junction (rectifier). Charge carriers: holes (p-type) and e- (n-type)

© 2008 Brooks/Cole 10

Solar panels employ a p-n junction

A photovoltaic cellphotovoltaic 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

typical metalR

esi

stan

ce

Temperature (K)

superconductor

Tc

critical T

0Wrong! Should approach

zero resistance!

Why are super conductors useful?

© 2008 Brooks/Cole 12

Metal Tc (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

Nb3Sn alloy 18.1

YBa2Cu3O7 90.

LaBa2Cu3Ox 35.

He(l) boils at 4.2 K

Many He(l)-cooled magnets use Nb-alloys

1st ceramicceramic superconductor (1986).

N2(l) boils at 77 K

“Y123” found 4 months later.

Highest Tc so far…Hg0.8Tl0.2Ba2Ca2Cu3O8.23 138.

Some alloys are better…

Superconductors

© 2008 Brooks/Cole 13

Levitating train can travel at >300 mph!