1 / 23

s b

E. s *. s b. E. Looking only at this region in the Rectangle:. We generated a Band Diagram. If we include the relative number of orbitals, we make a Density of States DOS Diagram. We generated a Band Diagram.

george-wiley
Download Presentation

s b

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. E s* sb

  2. E

  3. Looking only at this region in the Rectangle:

  4. We generated a Band Diagram If we include the relative number of orbitals, we make a Density of States DOS Diagram

  5. We generated a Band Diagram If we include the relative number of orbitals, we make a Density of States DOS Diagram

  6. We do the same thing again, starting with isolated atoms, Then turn on the bonding, then increase the number of interactions.

  7. P Mn Mn Polymeric unit P

  8. An actual example, calculated using an M.O.theory %Mn in orbital (state) %P in orbital (state) P Mn Mn Polymeric unit P

  9. Using Band Diagrams: Conductivity • Conductivity - in two flavors • 1. Electronic conduction - electrons move • typical of metals; • example: Cu and Al very good • conductivity “predicted” by band diagrams • 2. Ionic conduction - ions move • requires “ionic” material • requires defects: vacancy and interstitial • (Schottky and Frenkel types) • example: AgI2 and HgM2I4

  10. Semi Conductor Insulator MOT analogies with Band Diagram - HOMO / LUMO and type of reactivity - Valence Band / Conduction band and - DE and Band Gap MetallicConductor Empty bands small band gap Large Band Gap conduction band no band gap valence band filled bands

  11. More typically simplified to show only “frontier” bands: MetallicConductor Semi Conductor Insulator conduction band Fermi level ef no band gap Large Band Gap small band gap ef ef valence band DE < 10 kJ/mol DE ~ 10 -100 kJ/mol DE > 400 kJ/mol

  12. How Defects Improve Semi-Conduction Pure Germanium Gallium-Doped Ge Ga more Electropositive: Adds “Orbitals” At Higher Energy With Fewer Electrons Pure Ge Band Gap small band gap Gallium-Doping creates positive holes, as an acceptor band: DE ~ 0.66 eV A p-type semi-conductor

  13. How Defects Improve Semi-Conduction Arsenic-Doped Ge Pure Germanium As is more Electronegative: Adds “Orbitals” At Lower Energy Partially Filled with Electrons small band gap Pure Ge Band Gap Arsenic-Doping creates negative holes, as a donor band DE = 0.66 eV An n-type semi-conductor

  14. How Defects Lead to Devices PN Junctions = Diodes n-type p-type Fermi level in n-type semi-conductor is at higher energy than for the p-type: Spontaneous flow of electrons in one direction only. ef small band gap ef Directional Flow of electrons --> current goes in one direction only

  15. In a pn junction, current spontaneously flows in one direction

  16. How Defects Lead to Devices Band Gap threshold can be exceeded by: energy as light - photoconductivity devices: - photocells, photovoltaic cells (GaAs) - solar cells (Si) - pn-junctions with suitable ef make Light Emitting Diodes (LED) energy as heat – thermoconductivity devices: - thermistors

  17. How Defects Lead to Devices: Photocopy (Xerox) Process (photolithography) - uses photoconductivity of Selenium Se paper w/ image Se Ink (toner) Se

  18. How Defects Lead to Devices: Thermochromic Materials - example based on HgM2I4 materials

  19. Replace S with I, Zn (at vertices) with Hg Zn (in middle) with Ag Prototype Cubic ZnS (zinc blende), two adjacent cells Replace S with I, Zn (at vertices) with Hg, Zn (in middle) with Cu

  20. How Defects Lead to Devices: Thermochromic Materials - example based on HgM2I4 materials - adding energy as heat creates defects Cu(+) vacancies (Schottky defects) and interstital sites (Frenkel defects) - defects change band gap, change color, change conductivity

More Related