Electricity and Magnetism > Semiconductors and Tubes
 DCS# 5M10.xx
LED IN LIQUID NITROGEN
 
APPARATUS
yellow or red LED
 202-16-E3
power supply or battery
 202-08-D
dewar or clear cup
 214-01-C6
liquid nitrogen
 Science Center dock
tongs
 214-01-D6

DESCRIPTION
Show the temperature dependence of the band gap energy by cooling an LED in liquid nitrogen and observing the change in emitted wavelength.

Connect the LED to a 9 V battery in series with a 1 kohm resistor.  Dim the room lights.  Immerse the LED in liquid nitrogen and the red LED turns orange, and the yellow one turns green.  When the LED is cooled, the interatomic distances decrease, and the band gap increases, so the emitted wavelength decreases.

This works for LEDs without dopants, e.g. AlGaAs, where the band gap energy determines the emitted wavelength.  For doped LEDs, where there are energy states within the band gap, the color change can go the other way.

Use the document camera or a clear cup so the LED can be seen by the class.
REFERENCE

J. Chem. Educ., 1992, 69 (2), p 151