into a hole as it moves from the conduc- tion band to the valence band, with both the hole and the electron los- ing their charge. Because of the recombination of holes and electrons around the p-n junction, a barrier forms as the charges are depleted, which prevents the additional migration of electrons and holes across the junction. The only method suitable to enable additional migration across the p-n junction is to apply energy to the material. As energy is applied, this action results in holes and electrons continuing their recombination, and energy is emitted. When applicable material is used, the energy emitted will be in the form of light, providing for creation Light Sources and Detectors of the LED. Thus, an LED represents a p-n semiconductor with applica- ble doping to emit light when a forward bias is applied to the semicon- ductor. Figure 4.5 generally illustrates the application of a positive voltage across an appropriately doped p-n semiconductor to generate light. Note that the term forward bias in Figure 4.5 refers to the connection of the negative-battery terminal to n-type material and the positive-battery ter- minal to the p-type material. This action results in the injection of elec- trons into the n-type material and the creation of electrons in the p-type material by the extraction of electrons from that material. As a result of this electrical action, both electrons and holes move toward one another across the depleted area of the junction. As they combine, their energy is emitted in the form of light as shown in the lower portion of Figure 4.5. Because current must be continually supplied to maintain an excess of carriers for recombination, the toggling of the forward bias on and off controls the generated light. Thus, by varying the forward bias on and off, you are now able to generate light and turn it off. By varying the for- ward bias in tandem with the value of electrical pulses, it becomes possi- ble to convert electrical signaling into optical signaling. One of the key limitations 