|This is a
Free simulation software that you can download and run on your Windows
Below is a screen shot of the LED simulation (LED stands for light emitting diode). If you decide to download and install the simulation program you will be able to adjust the knobs and buttons you see below.
The software screen shot below is showing you a PN junction or a "DIODE". The software will let you adjust the input voltage to allow the recombination of the electrons and holes. Note what happens to the photon when you adjust the band gap energy.
This free software simulation has much to do with band gap theory. I learned about this in my college quantum mechanics class at Arizona State University.
Note: The photon color is not working yet so that it corresponds with the band gap energy that the user sets.