Solid State Physics and Planck's Constant with an
LED
LED
This experiment is somewhat anachronistic in that the discovery of the
photoelectric effect stands at the foot of a huge number of advances that enabled the
invention of the LED. However, this lab gives a clear demonstration of the photoelectric
effect, and a gentle introduction to solid-state physics, for under ten dollars down. You
could build this one at home!
Prelab questions
How big is the energy gap in a yellow LED? Why are blue LED’s hard to make?
Briefly summarize the procedure you will follow, including the ranges and variables you
will record.
Go look at the circuit for this experiment in the lab. Why did use such a ridiculously
big resistor as the shunt (R2 in the circuit above)?
Procedure
You will take V-I curves for each color of LED, and use the curve to determine the band
gap of the LED, among other parameters. The band gap and the wavelength of the emitted light are related by the
Planck-Einstein equation,
Note important features in
your data set: for instance, the voltage at which the LED first becomes visible. Make sure
to take data over the appropriate range. Do not exceed the maximum current of the LED! Get
your TA to show you how to use the current limit of the power supply. Your analysis should
go into some depth on the solid-state physics of the LED; this is a chance to really
understand the physics of semiconductor devices.
Points to Consider During Your Analysis
In lieu of the classical expectations normally required for a lab, you
should explain why the method outlined in the paper by O’Connor is wrong.
and the wavelength 
of the emitted light are related by the
Planck-Einstein equation,
