 Hello and welcome to the lecture on LED structures. Learning outcome of this session. By the end of the session student will be able to illustrate different structures of light emitting diodes. You may pause here the video and enlist the desirable properties of LED as a optical source. I hope you have enlisted the desirable properties. High intensity or radiance. High efficiency. Low power consumption. High reliability. Narrow spectrum. Less response time are some of the desirable properties of the LED as a optical source. The LED structures. Mainly there are five major types of LED structures and these are planar LEDs, dome LEDs, surface emitter LEDs, edge emitter LEDs, super luminescent LEDs or SLDs. We will discuss each structure in detail. Out of these five LED structures only surface emitter LED, edge emitter LED, super luminescent LED have found extensive use in optical fiber communications as these are highly intense and have high efficiency and reliable in comparison with the other two types of structures. The other two structures that is planar LED and dome LEDs find more applications as cheap plastic encapsulated visible devices for use in areas as intruder alarms, TV channel changers and industrial counting etc and so on. The first LED structure is planar LED. The planar LED is the simplest of the structures that are available and fabricated by either liquid or vapor phase epitaxial processes over the whole surface of gallium arsenide substrate. This involves a p-type diffusion into n-type substrate in order to create the junction as shown in figure number 1. Forward current flows through the junction gives lamratian spontaneous emission and the device emits light from all the surfaces. However, only a limited amount of light escapes the structure due to total internal reflection and therefore radiance is low. This low radiance results in non-intense emission which is not a desirable property of an optical source. Next LED structure is dome LED. The structure of a typical dome LED is shown in figure number 2. A hemisphere of n-type gallium arsenide material is formed around a diffused p-type region. The diameter of the dome is chosen so that maximum amount of internal emission reaching the surface within the critical angle of gallium arsenide and air interface. Hence, this device has a higher external power efficiency than the planar LED. However, the geometry of the structure is such that the dome must be far larger than the active recombination area which gives a greater effective emission area and thus reduces the radiance resulting in non-intense emission which does not meet the desirable property of an optical source. The next LED structure is surface emitter LEDs. A method for obtaining high radiance is to restrict the emission to a small active region within the device. The technique pioneered by Boris and Dawson with homostructure devices was to use an etched well in a gallium arsenide substrate in order to prevent heavy absorption of the imitated radiations and physically to accommodate the fiber. These structures have a low thermal impedance in active region allowing high current densities and giving high radiance emission into the optical fiber. The structure of a high radiance etched well double hetero junction surface emitter for 0.8 to 0.9 micrometer wavelength band is shown in figure number 3. In figure number 3, you can observe etched well area. Furthermore, considerable advantage may be obtained by employing double hetero junction structures giving increased efficiency from electrical and optical confinement as well as less absorption of the imitated radiations. This type of surface emitter LED has been widely employed within optical fiber communications. The internal absorption in this device is very low due to larger band gap confining layers and the reflection coefficient at the back crystal phase is high giving good forward radiance. The emission from the active layer is essentially isotropic although the external emission distribution may be considered lambretian with a beam width of 120 degrees due to refraction from a high to a low refractive index at gallium arsenide and fiber interface. The next LED structure is edge emitter LEDs. Another basic high radiance structure currently used in optical communications is a strip geometry double hetero junction edge emitter LEDs. This device has a similar geometry to a conventional contact strip injection laser which is shown in figure number 4. It takes advantage of transparent guiding layers with a very thin active layer of 50 to 100 micrometers in order that the light produced in active layer spread into transparent guiding layers to reduce self-absorption in the active layer. The consequent wave guiding narrows the beam divergence to a half power width of around 30 degrees in the plane perpendicular to the junction. However, the lack of waveguiding in the plane of junction gives a lambretian output with a half power width of around 120 degrees which is shown in figure number 4. In figure number 4 you can observe the angle of 120 degrees. Most of the propagating light is emitted at one end phase only due to reflectors on the other end phase and anti-reflection coating on the emitting end phase. The effective radiance at the emitting end phase can be very high giving an increased coupling efficiency into small numerical aperture fibers compared with the surface emitters. However, surface emitters generally radiate more power into air than the edge emitter LEDs. Since the emitted light is less affected by the reabsorption and interfacial recombinations, comparisons have shown that edge emitter LEDs couple more optical power into low numerical aperture fibers than surface emitting LEDs whereas opposite is true for large numerical aperture fibers. The enhanced waveguiding of the edge emitter LEDs enables it to couple 7.5 times more power into low numerical aperture fiber than a comparable surface emitter LED. The last structure is super luminescent LEDs or referred as SLD. Another device geometry which is providing sufficient benefits over the both surface emitter LEDs and edge emitter LEDs for communication applications is the super luminescent diode or SLDs. This device offers advantages like high output power, directional output beam, narrow spectral line width. All of these are the desirable properties of LED as a optical source and all of these advantages prove useful for coupling significant optical power into optical fiber in particular to the single mode fibers. Figure number five shows the construction of aluminum gallium arsenide contact strip super luminescent LED structure. It may be observed that the structure is very similar to those of edge emitter LEDs. For operation the injected current is increased until stimulated emission and hence amplification occurs. But because there is a high loss at one end of the device no optical feedback takes place. Therefore, although there is amplification of the spontaneous emission no laser oscillation builds up. However, operation in the current region for stimulated emission provides gain causing the device output to increase rapidly. This increase is with increase in drive current due to what is effectively single pass amplification. High optical power can therefore be obtained together with a narrowing of the spectral width which also results from the stimulated emissions. Although incoherent optical power output from the SLD can approach that of the coherent output from injection lasers. The required current density is substantially higher by around a factor of three times. Necessating high drive currents due to the long device active lens that is due to larger areas. These are the references. Thank you.