Modeling LEDs

We will now use the LED Constructor computer program to understand how these two materials are combined to construct an LED.

Notice that on the left of the screen is an illustration of 6 different LEDs and a version of the circuit apparatus that we used when we explored the properties of LEDs. On the right of the screen are the two blocks of material that make up the LED chip with an energy scale and output spectrum illustrated below the blocks.

In this program, we can:

• Select an LED and drag it to the power supply (LED socket).
• Create an energy band diagram for each of the two semiconductor blocks that make up each LED.
• Add donors or acceptors to each semiconductor block with a click of the mouse inside each block and thus create the LED chip.
• Control the voltage applied across the LED by moving the slider.

When the voltage is appropriate, transitions (represented by a vertical arrow) will occur and light will be emitted. A spectrum will appear below the energy scale.

Drag a LED of your choice to the LED socket. The energy bands that appear on both sides of the energy scale represent the bands and gap for materials associated with this LED.

Notice that that energy of the energy gaps of both blocks are the same to represent that the LED is constructed of two blocks of the same semiconductor.

Record the color of the LED you are using and the energy (in eV) of the energy gap.

You make like to select other LEDs and compare the energy gaps.

Now, click on the Add Impurities button. This places acceptors in the left block and donors in the right block. To see the effect of this process on the energy bands, click the Merge button to bring the two semiconductor blocks together and, thus, create a "chip" that makes up the LED.

Notice that some bands of both semiconductor materials are shaded darker than others. This shading indicates that the majority of electrons have energies associated with the valence bands. Electrons are naturally found in these bands because they seek the lowest possible energies.

Before you continue using the LED Constructor program answer the following questions:

1. How does adding acceptors --- atoms with fewer electrons --- affect the energy bands of the left block?                                                                                            
2. What is the effect on the energy bands of adding donor atoms to the right block?              
3. Do the sizes of the energy gaps for either semiconductor block change as donor or acceptor atoms are added?                                                                                                   
4. A material that has donor atoms has more than one with acceptors. When these two materials are joined together, the electrons can move from one material to the other. Which way would you expect the electrons to move? Why?

As you see, the bands and gaps shift from their usual energies. This shift is a result of the movement of electrons from one material to the other.

Make a sketch of the energy diagram for your LED and then select a different LED, add impurities and merge. Compare the two energy diagrams of your two LEDs.

How are they similar?
How are the different?

The situation that is now represented on the screen is equivalent to an LED that is not connected to an energy source. The energy diagram for the merged material is similar to that illustrated below:

In this situation extra electrons are still available on the right (donor) side. However, they cannot change energies from the conduction band to valence band in the donor side. All of the valence energies are already filled with electrons. We must move the electrons to the other material where the change in energy is easy.