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Code Number: 7A70.50
Demo Title: Cool a PN Junction - LED Band Gap
Condition: Good
Principle: Band Gap
Area of Study: Modern Physics
Equipment:
Identical LED's with wires, 3 volt battery, liquid nitrogen, 300 ml Dewar.
Procedure:
Plug in two LED's and show that they have the same visible frequency or color. Dip one of the LED's into liquid nitrogen until it is as cold as possible. Remove and place beside the other LED. Visible color change should be apparent.
Yellow LED Color Change vs. Voltage
Attach a single 8mm yellow LED to the Hewlett-Packard Model 721 power supply with the current range set to the 225 ma scale.
Turn up the voltage until the current goes well past 25 ma. The LED will change color from yellow to a reddish brown.
Dunk the LED into the liquid nitrogen for a short time and observe the color shift to orange.
Dunk the LED into the liquid nitrogen for a longer time and observe the color shift go past orange to yellow, and then to yellow green.
If you then turn down the voltage at the yellow green stage so that the current goes below 20 ma, you will see that the color shifts to a bright green. Allow the LED to warm up at this stage and you will see the color shift back to yellow.
References:
- Gorazd Planinšič and Eugenia Etkina, "Light-Emitting Diodes: A Hidden Treasure", TPT, Vol. 52, #2, Feb. 2014, p. 94.
- John W. Jewett, "Get The LED Out", TPT, Vol. 29, # 8, Nov. 1991, p. 530.
- J. A. Davis and M. W. Mueller, "Temperature Dependence of the Emission from Red and Green Light Emitting Diodes", AJP, Vol. 45, # 8, Aug. 1977, p. 770.
- John B. Kwasnoski, "A Laboratory Investigation of Light-Emitting Diodes", AJP, Vol. 40, # 4, Apr. 1972, p. 588.
- Charles Day, "Light-Emitting Diodes Reach the Far Ultraviolet", Physics Today, July 2006, p. 15.
- Charles Kittel, Introduction to Solid State Physics 5th Edition, John Wiley & Sons, Inc.
- "Photonic-Crystal Advances", How it Works - Science Supplement, Spring 2000, pp. 319.
- T. D. Rossing, C. J. Chiaverina, "# 6.8, Light-Emitting Diodes", Light Science, Physics and Visual Arts, p. 140.
- George C. Lisensky, Rona Penn, Margret J. Geselbracht, and Arthur B. Ellis, "Periodic Properties in a Family of Common Semiconductors: Experiments with Light Emitting Diodes", Journal of Chemical Education, Vol. 69, #2, Feb. 1992, p. 151.
- Keithley Instruments, "Physics Experiment # 4: Energy Gap in A Semiconductor, Series I.
- Joseph Florentine, David Maiullo, "Shifting Bandgaps: LEDs in Liquid Nitrogen", Rutgers Physics Lecture Hall.
- Tap-L Conversations, Oct. 6, 2008.