Homework on Wide Bandgap Semiconductors (solutions)

Feel free to work on the homework in groups. The work you hand in, however, should reflect your understanding of the material and be in your own words.Students who turn in identical (or close to identical) homework assignments will be asked to explain their answers orally to the TA or prof.  A student who cannot explain how he or she arrived at a given answer will be charged with academic dishonesty.

You should  justify all of your answers for full credit.
 

1.	Explain the similarities and differences between LEDs and diode lasers, using terms such as monochromatic, stimulated emission, electron-hole recombinations, pumping, and population inversion. LEDs and diode lasers both use electron-hole recombinations to generate monochromatic light. LEDs require nothing further; lasers use stimulated emission - in which a photon of the correct wavelength passes by an excited electron, causing another identical photon to be emitted - to generate coherent light. Lasers need to provide additional pumping energy, pushing a large number of electrons into the conduction band - a condition known as population inversion - in order for stimulated emissions to be much more common than spontaneous emissions.
2.	Dr. Schowalter discussed how materials are really catagorized into only two groups:  metals (conductors) and insulators.  Why don't semiconductors make a third group?  Where do they fit into the catagorization and why do we treat them differently? Semiconductors are a subclass of insulators, having a filled valence band.  (Conductors have a partially filled valence band.)  A material is considered a semiconductor if its conductivity can be controled, e.g., by doping.)
3.	Thanks to the wonderful education you received at Rensselaer, you land a job as director of marketing for Dr. Schowalter's wide bandgap semiconductor firm (which has grown into a market leader in the industry).  You solicit ideas from your minions for advantages of wide bandgap semiconductors that you should include in promotional materials.  Explain why each of the following suggestions should or should not be included in your brochures. Wide bandgap semiconductors are necessary to produce white light with LEDs (if this is included in your brochure, indicate what some of the advantages/applications are of white LED light). Wide bandgap semiconductors are advantageous because they can conduct with a smaller applied voltage and thus generate less heat when used in chips (if this is included in your brochure, indicate why low heat is important). Wide bandgap semiconductors are advantageous to use in optical storage devices (if this is included in your brochure, indicate the connection between wide bandgap semiconductors and optical storage). Wide bandgap semiconductors can be used at higher temperatures than smaller-bandgap semiconductors (if this is included in your brochure, indicate the advantages of operating at higher temperatures and the connection between maximum operating temperature and bandgap). Point 1 is true - white light is composed of several different frequencies, and you need wide bandgap semiconductors to generate the higher visible frequencies (i.e. blue light). White LED light is useful for basic interior lighting, where the extremely long life and low power consumption of LEDs make them attractive. Point 2 is false - wide bandgap semiconductors require a greater applied voltage to get electrons up into the conduction band. Point 3 is true - again, WB semiconductors allow for shorter wavelength light. This in turn will permit smaller spot sizes to be used on optical media, allowing a great data density. Point 4 is true - the larger bandgap means that thermal energy will not excite as many electrons across it, allowing finer control of the number of charge carriers.