Tuesday, November 27, 2012

LED: Energy Bands of a Semiconductor

Purpose: To use  an understanding of p-n junctions in an LED light to measure plank's constant.

Introduction: A light-emitting-diode is a semiconducting diode which is used for the emission of light. A voltage difference excited electrons across a band gap and when they return to the lower energy band a photon is emitted. We can use the emitted wavelength and voltage to calculate plank's constant.

Equipment:

  • Voltage source
  • Alligator Clips
  • Variable Resistance
  • Volt Meter
  • Various colored LED lights
  • A Diffraction Grading
  • 2 Meter Sticks
Procedure: Set up the circuit shown in the picture to the side where the source is attached to your variable resistance then to the LED. Make sure to keep some resistance in the circuit at all times, and to make sure your LED is connected correctly as voltage will only flow in one direction.
  








   Next increase the resistance until the light is barely visible (we want the minimum voltage required to light the bulb (excite the electrons). Record the  voltage across the LED. Do this for various colors.
     
    The second part of this experiment requires the determination of wavelengths. To do this we set up a spectrometer (as is described in my Color and Spectra Lab). Using this set up sine the bulb at the diffraction grading from the end on the meter stick and with one person at the diffraction grading direct another person the spot of most intensity on that color as seen on the second meter stick. Use this to determine the wavelengths as shown below.







     The LEDs will give a mixture of colors for the spectra unlike the isolated gasses.



Data and Analysis:

The error with each is as follows:
     Green                     12.6%
     Blue                       18.4%
     Yellow                    22.2%
     Red                        20.6%

The values of Plank's constant are fairly close. At our best we are 12.6% low of the accepted value. There is a systematically low error for this lab which agreed with the rest of the class but we are very close.

The green was the most accurate and this is because the human eye is most sensitive to the wavelengths corresponding to green.

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