Chapter 20 section 20 3 Semiconductors and superconductors

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Chapter 20, section 20. 3 Semiconductors and superconductors © 2008 Brooks/Cole 1

Chapter 20, section 20. 3 Semiconductors and superconductors © 2008 Brooks/Cole 1

Metallic Bonds: The “electron” sea model Valence electrons can move anywhere within the metal

Metallic Bonds: The “electron” sea model Valence electrons can move anywhere within the metal These electrons are delocalized (similar to resonance) © 2008 Brooks/Cole 2

To understand metallic bonding and electrical conductivity, we need to re-visit molecular orbital theory.

To understand metallic bonding and electrical conductivity, we need to re-visit molecular orbital theory. © 2008 Brooks/Cole 3

Formation of bands of valence orbitals in alkali metal Number of atoms e- in

Formation of bands of valence orbitals in alkali metal Number of atoms e- in lattice energy levels 1 2 3 A metal behaves like a giant molecule Valence orbitals extend over the entire metal 12 many empty filled © 2008 Brooks/Cole Electrons in partially filled valence band are free to move throughout the metal 4

What if the valence band is full as in Mg? Electrons can be excited

What if the valence band is full as in Mg? Electrons can be excited to empty “conduction band”! © 2008 Brooks/Cole 5

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

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 6

Doped semiconductors: n-type and p-type Si doped with As or P Extra electron is

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 7

Movement of holes requires concerted movement of large numbers of electrons Hole (+) Equivalent

Movement of holes requires concerted movement of large numbers of electrons Hole (+) Equivalent to Electron (-) Which type of semiconductor is a better conductor? Why? N-type. Holes are less mobile than electrons © 2008 Brooks/Cole Analogy courtesy of Dr. Joe Lomax 8

Current flows in only one direction through p-n junction! A p-n junction (rectifier) Charge

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 9

Solar panels employ a p-n junction e- A photovoltaic cell Light drives e- around

Solar panels employ a p-n junction e- A photovoltaic cell Light drives e- around an external circuit © 2008 Brooks/Cole 10

Superconductors Metals are better conductors at low T. al t e lm Resistance Why

Superconductors Metals are better conductors at low T. al t e lm Resistance Why are super conductors useful? a c i p ty r o t c du n o rc e sup critical T Wrong! Should approach zero resistance! 0 Tc Temperature (K) Superconductors have zero resistance (are perfect conductors) at some (low) T. © 2008 Brooks/Cole 11

Superconductors Several metals are superconductors at low T Metal Aluminum Gallium Tin Mercury Lanthanum

Superconductors Several metals are superconductors at low T Metal Aluminum Gallium Tin Mercury Lanthanum Lead Nb 3 Sn alloy La. Ba 2 Cu 3 Ox Tc (K) 1. 15 1. 10 3. 72 4. 15 4. 9 7. 2 18. 1 35. YBa 2 Cu 3 O 7 90. 138. Hg 0. 8 Tl 0. 2 Ba 2 Cu 3 O 8. 23 © 2008 Brooks/Cole Some alloys are better… 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 “Y 123” found 4 months later. Highest Tc so far… 12

Levitating train can travel at >300 mph! © 2008 Brooks/Cole 13

Levitating train can travel at >300 mph! © 2008 Brooks/Cole 13