Optoelectronic Semiconductor Devices Principals and Characteristics

4.1 TWO BASIC LED STRUCTURES 
The basic structures of various kinds of LEDs are illustrated in Figure 
24.
 

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a) Burrus type; 
b) surface-emitting type (double heterostructure type); 
c) surface-emitting type (homojunction type). 
[5]
 
Figure 24.: Cross-section illustration of the basic surface-emitting types of LEDs:
In LEDs used in optical fiber communication systems, efficient spontaneous emission originating from 
the injection excitation is favorable to reduce input power, and thus a pn-heterojunction - usually DH - is 
used. 
A pn-homojunctions (as well as pn-heterojunctions) is still used in LEDs emitting red or yellow light 
because it is easy to fabricate. 
The light originating in the spontaneous emission process is emitted from the light-emitting region (active 
layer) in all directions. Consequently, several structures restricting the emitted light to a certain directions 
have been developed. These LED structures are basically divided into two groups: The surface-emitting 
and Edge-emitting types. 
4.1.1 
SURFACE-EMITTING LED
 
The surface-emitting LEDs emit the light in a direction perpendicular to the pn-junction plane. Figure 
24.
shows three surface-emitting types of LEDs. 
The general idea is that a narrow active layer is sandwiched between two cladding layers. Because of 
heterojunctions on either side of the active layer, injected carriers are confined within the thin active 
region. The thick substrate is used to provide mechanical support and electrical contacts. 
In the Burrus type, the emitted light is absorbed in the substrate. If the wavelength (photon energy) of the 
emitted light is shorter (higher) then the fundamental absorption edge (band-gap energy) of the substrate, 
most of the emitted light is absorbed in the substrate. In such case, a part of the substrate corresponding to 
the light-emitting region is etched off to prevent the absorption. 
The light-emitting region is usually limited in order to keep the operating current low and, for LEDs used 
in optical fiber communication systems, to facilitate the coupling of the light to a fiber. The emitting 
region is limited by confining the injected current to a part of the electrode. The electrode is in contact 
with a cap layer, usually 30 to 50 µm in diameter, and is limited by a dielectric film such as SiO
2
. 
If the emitted light is transparent in the substrate, the substrate can be chemically or physically etched 
down to lens structure to improve the light-emitting pattern. 

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a) monolithic-lensed type;
b) hemispherically shaped type;
c) edge-emitting type with a guiding layer;
d) edge-emitting type with local emitting area.