h lasers. Nevertheless, LEDs are relatively inexpensive and serve well for short transmission dis- tances. This explains why most early fiber-optic transmission systems were based on the use of LEDs as a light source. Operating at a wave- length of 850 nm for use with multimode optical fiber, early LED-based systems were capable of transmitting at rates of up to 140 Mbits/s. Unfor- tunately, such systems required repeaters every 10 to 15 km. With the development of low-cost lasers and the migration to single-mode fiber by most communications carriers, the use of LEDs is now relegated primari- ly for LAN and other short-distance applications. Today the upper throughput limit of LED-generated signals is approximately 300 Mbits/s. In comparison, lasers routinely operate at 10 Gbits/s and in field trials now operate at 40 Gbits/s. Now that we have a general appreciation for LEDs versus lasers, let's examine how the former operates. Figure 4.2 Light generated by an LED is incoherent and spewed in all directions. Chapter Four Operation Light can be generated in several ways. The familiar lightbulb represents incandescent light since optical radiation results from the temperature applied to a filament. In comparison, an LED represents light generated by the electronic excitation of a material, a term referred to as lumines- cence. To understand how semiconductor chips, including LEDs and lasers, operate, let's review some basic physics, including the structure of atoms and the p-n semiconductor junction, to see how a semiconductor becomes capable of generating light. THE COMPOSITION OF AN ATOM Figure 4.3 illustrates the com- position of an atom. An atom consists of an inner nucleus surrounded by electrons that circle the nucleus. That circle is referred to as a shell. Each shell has a maximum number of electrons, ranging from two in the inner shell (K) to 32 in the outer shell (N). The outer shell is referred to as the valence shell or valence band, and represents the shell that is involved in chemical bonding, which pe