7A70.50 - Cool a PN Junction - LED Band Gap

Code Number:
Demo Title:
Cool a PN Junction - LED Band Gap
Band Gap
Area of Study:
Modern Physics
Identical LED's with wires, 3 volt battery, liquid nitrogen, 300 ml Dewar.

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.

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