2. Putting Electrons to Work Doping and Semiconductor Devices

3. N-type semiconductors N-type semiconductor is doped with a material having extra valance electrons Result is filled energy states in the band gap just below the conduction band These electrons can easily gain energy to jump to the conduction band and move through the material

4. P-type semiconductors P-type semiconductor is doped with a material having fewer valance electrons Result is “holes”, or empty energy states in the band gap just above the valance band Since no single electron travels through the material, we describe the charge carrier as a positive hole moving the other way

5. Doped Semiconductors Energy n-type p-type donor level acceptor level Put them together?

6. p-n junction n-type p-type Energy depleted region (electric field) ++++++ + + ----- - - -

7. P-n junction As more electrons from the n-side combine with holes from the p-side, each additional combination adds to the potential difference across junction This can be envisioned as shifting the energy bands, making it harder for electrons to travel across the barrier

8. p-n junction n-type p-type Energy depleted region (electric field) ++++++ + + ----- - - - VoVo

9. P-n junction Originally both p and n sides are electrically neutral Electrons in n side see holes in p side and combine Second electron needs add’l energy to get over charge barrier – can represent as rise in energy levels of p section

10. Forward Biasing Eventually, the potential difference is so large, electrons cannot travel across it without gaining energy Applying a forward bias decreases the potential difference so current can flow

11. Reverse Biasing Applying a reverse bias will increase the barrier rather than decreasing it, so no current flows

12. Light-emitting Diode When an electron loses energy to recombine with a hole, it can emit that lost energy in the form of light. This light always has roughly same E, so LEDs emit small range of wavelengths  This light-emitting property of p-n junctions can be utilized to create a laser  Be sure to come to class to hear Dr. Schowalter say...

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14. How does a semiconductor laser work?

15. Absorption and Emission E o E 1 photon out photon in

16. Stimulated vs. Spontaneous Emission We can now derive the ratio of the emission rate versus the absorption rate using the equilibrium concentrations of photons and excited atoms: l Derived in 1917 by Einstein. Required stimulated emission. However, a “real” understanding of this was not achieved until the 1950’s.

17. Laser needs a Population Inversion

18. Biased junction n-type p-type depleted region (electric field) Negative bias photon out

19. History of Lasers First operating Laser in 1960 (Maser in 1958) –Simulated emission concept from Einstein in 1905 –Townes (1964) and Schawlow (1981) First semiconductor injection Laser in 1962 –First was Robert Hall (GE) but many competing groups –Year before he had argued it was impossible

20. Violet Laser Diode Currently costs about $2000 apiece!

21. Nichia Laser Diode Epitaxial Lateral Overgrowth material 10,000 hours operation! 10 mW CW 405 nm

22. Before the next class,... Start Homework 20 Read Chapters 4 Turton. Do Reading Quiz