1. © 2008 Brooks/Cole 1 Chapter 20, section 20.3 Semiconductors and superconductors

2. © 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)

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

4. © 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

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

6. © 2008 Brooks/Cole 6 So why don’t all solids conduct electricity? Metals, Semiconductors & Insulators Silicon and germanium form network solids similar to diamond but they are semiconductors.

7. © 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

8. © 2008 Brooks/Cole 8 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

9. © 2008 Brooks/Cole 9 Current flows in only one direction through p-n junction! A p-n junction (rectifier) A p-n junction (rectifier). Charge carriers: holes (p-type) and e - (n-type)

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

11. © 2008 Brooks/Cole 11 Metals are better conductors at low T. Superconductors have zero resistance (are perfect conductors) at some (low) T. Superconductors typical metal Resistance Temperature (K) superconductor TcTc critical T 0 Wrong! Should approach zero resistance! Why are super conductors useful?

12. © 2008 Brooks/Cole 12 MetalT 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 alloy18.1 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 ceramic 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

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