How Do LEDs Emit Light?

In order to convert electrical current into light, an LED must have a p-type semiconductor in contact with an n-type semiconductor.This combination of the two types of semiconductors is known as a p-n junction, or a diode. When a p-n junction is placed in a circuit with an external power source (e.g., a 9 V battery), electrons from the power source flow to the diode and change the arrangement of electrons in the diode (Figure 10). How does this lead to the emission of light in the LEDs used to test conductivity in this experiment?

Recall that the p-type semiconductor (Figure 8) has extra space for electrons in its valence band, and no electrons in its conduction band, while the n-type semiconductor (Figure 9) has a full valence band (no space) and extra electrons in its conduction band. If the circuit is constructed such that electrons flow into the n-type side of the p-n junction from the power source (Figure 10), they will occupy the conduction band, since there is no space in the valence band of an n-type semiconductor. As electrons continue to come into the conduction band, they will be pushed to the p-type side of the p-n junction, which has more space to hold electrons (you can think of the "positive" side attracting the negatively-charged electrons). The electrons go into the empty conduction band of the p-type side, since they already occupy the higher-energy band in the n-type side. However, once the electrons are in the higher-energy band of the p-type side, they will fall to the lower-energy band if there is space available for the electrons to occupy in the valence band. Electrons falling from the higher-energy band of orbitals (conduction band) to the lower-energy band of orbitals (valence band) in the p-type semiconductor results in the atoms going from a higher-energy state to a lower-energy state (i.e., becoming more stable). As the electrons cross the band gap, energy related in magnitude to the size of the band gap is released in the form of light.

Figure 10 The diagram on the left schematically shows the path of electrons moving through a circuit containing a p-n junction. Electrons flow from the negative (shown in black) pole of the battery to the n-type semiconductor, where they occupy the higher-energy (conduction) band. The electrons then move into the conduction band of the p-type semiconductor and fall into the empty orbitals of the valence band, which releases energy in the form of light. The electrons then move through the wire back to the positive pole (shown in copper) of the battery, and re-circulate. (Note: This picture has been simplified to follow the discussion presented here. A more complete picture would show the two doped semiconductors in equilibrium, in which the band gaps are "bent" and a potential energy barrier to electrons moving from the n-type to the p-type semiconductor must be overcome (using the voltage from the battery; known as "biasing"). For more information on bending in diodes, see Elllis et. al in the References.) The diagram on the right depicts a circuit composed of an LED, a resistor, and a power source (battery). The flow of electrons through this circuit is the same as that described above. The resistor in the circuit (which is not shown on the left for ease of viewing) is necessary to limit the current so that the LED does not "burn out" by receiving too much current from the battery. Please click on the pink button below to view a QuickTime movie showing the flow of electrons through the circuit, and the light emitted by the LED. Click the blue button below to download QuickTime 6 to view the movie.

The color of the light emitted depends on the size of the band gap. The LEDs used in the Experiment are made of a combination of semiconducting materials specially chosen to have the right size band gap for yellow light to be emitted. LEDs that emit red light, which are used in many digital alarm clocks, have a different-size band gap, and therefore a different amount of energy is released in the form of light (Figure 11). (Later in the semester you will learn that light of different colors has different energies.) LEDs that emit infrared rather than visible light are common in remote controls for televisions and stereos.

Figure 11 The color of light emitted by an LED depends on the size of the band gap in the doped semiconductors. For instance, LEDs that emit red light have a smaller band gap than LEDs that emit yellow light.

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