LED Constructor

STEP 1: Selecting the Color of the LED

Click on one of the LEDs shown at the bottom left of the screen and drag and drop it into the LED holder in the circuit.  The energy bands of the material of the LED appear on the right.

STEP 2: Constructing the LED

Click the "Add Impurities" button on the top right of the screen.  The acceptor and donor atoms are added in the block on the left and right respectively.  These blocks represent two pieces of semiconductor material.  The energy bands below each block do not change, but the bands below the N side material, on the right (that has the donor impurities) becomes shaded dark, indicating an abundance of electrons on that side.  The P side material, on the left (that has the acceptor impurities) remains light shaded.
Then click the "Merge" button.  The two blocks as well as the energy bands on the P and N sides merge.  The merging causes an offset between the energy bands on the left and those on the right.  This difference corresponds to the energy barrier that electrons on the N side would experience in attempting to go to the P side

STEP 3: Applying a Voltage across the LED

After the LED has been constructed, the voltage slider in the circuit in the top left appears.  Dragging this slider to the right on the 0 to 10V scale, corresponds to turning a potentiometer in the circuit so that the corresponding voltage is applied across the LED.  You can also apply a voltage across the LED, by clicking the right arrow key on the keyboard.
Observe the change in energy band of the LED on the right.  In the default mode, the battery is connected so that the LED is forward biased i.e. the N side of the LED is connected to the negative side of the battery, and the P side to the positive side of the battery.  The energy bands on the N side rise to indicate an increase in energy of the electrons there.  When the applied voltage is sufficiently high the energy bands on the N side will match those on the P side.  At this stage electrons from the N side can migrate to the P side in the conduction band and then loose energy and drop into the valence band and emit light.  The LED lights up and its spectrum appears on the top right of the screen.

STEP 4: Reversing the Polarity of the LED

Clicking the "Flip" button is equivalent to reversing the polarity of the LED, so that LED if the LED is initially forward biased (which it is by default), it becomes reverse biased after the button is clicked.  Increasing the voltage of a reverse biased LED using the voltage slider will cause the difference in the energy band on the N side and P side to increase, so that the LED will not light up.

STEP 5 (Advanced): Building a Hypothetical LED

Build an LED with any given energy gap and learn the relationship between the energy gap, turn-on voltage, and spectrum of an LED. Click, drag, and drop the unknown LED from the array of LEDs at the bottom left of the screen into the LED holder.  This causes the energy bands to appear on the right.  Unlike the LEDs that are labeled with a particular color, that have fixed energy bands, the energy gap of this LED can be altered by clicking, dragging and dropping the valence (lower) energy bands on below either of the two blocks.  After the valence band on one side has been placed at a desired energy level, the valence band on the other side automatically adjusts to the same energy level, so that the energy gap is the same on both sides.  Next apply a voltage across this LED and observe the changes in the energy bands and spectrum.