Optoelectronic Semiconductor Devices Principals and Characteristics

Figure 25.: Cross-sectional views of some widely used LEDs: 
Because radiative recombination is more probable in direct-band-gap semiconductors than in indirect-
band-gap semiconductors, direct-band-gap materials are preferred for the light emitting layers.(Figure 
25.
). 
For typical surface-emitting LED designed for optical fiber communication purposes, the active area, 
defined for example by an oxide stripe, has a width of 20 to 50 µm, which matches the core diameter of 
typical multimode fibers. 
4.1.2 
EDGE-EMITTING LED
 
The structure is very similar to that of ILDs.(Figure 
26.
) 
Figure 26.: Cross-section illustration of the basic edge-emitting type of LEDs. 
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The light is emitted in a direction parallel to the pn-junction plane. The current is restricted, by a 
dielectric film, to within a stripe having a width less than several tens of micrometers and then the light-
emitting regions is formed along the current injected (stripe) region. 
The cladding layers are chosen to provide carrier confinement, as well as optical guidance. since the 
index of the cladding layers is slightly lower than that of the active layer, rays with a large incident angle 
are total reflected internally by the boundaries between the two layers. This means that light is trapped 
inside the high-index light-emitting layer and emerges only from the diode edge. 
Edge-emitting LEDs have an elliptical pattern. 
In some edge-emitting LEDs, current injection in the region near the rear facet is reduced by an 
electrically resistive region, which means that the region near the rear facet absorbs rather than generates 
light. 
This edge-emitting type of LEDs is used in the optical sources of fiber communication systems, but not 
often.