Optoelectronic Semiconductor Devices Principals and Characteristics

Figure 8.: Transverse band structures for two different separate-confinement heterostructures (SCHs):
In order to be able to compare the performances of various diodes, it is customary to use the threshold 
current density J
th
. 
When the injection current is less than threshold value, the radiation of the ILD is mainly broadband, 
incoherent, spontaneous emission. 
For current greater than the threshold current, the emission becomes mostly coherent and has a narrow 
spectral width. 
The first-generated ILDs were fabricated by diffusion techniques and had homojunctions. All layers in 
these diodes were made of GaAs. As we know properties of the diffusion techniques used in fabrication, 
one side of the active region was not well defined. Which means that the injected charge carriers were not 
confined in the narrow active region. The index difference ∆n between various layers was very small. J
th
of these ILDs was as high as 100kA/cm
2
. 

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Figure 9.: Schematic representation of energy band diagram, refractive index profile, and optical field 
distribution of homostructure, and single heterostructure LEDs and ILDs. 
[1]
 
The second generation laser diodes were made by liquid phase epitaxy techniques. The boundaries 
between layers were well defined. Since all layers were based on GaAs, the energy band-gap difference 
was very small and junctions were homojunctions. 
Because of the sharp junction interfaces, charge carriers were weakly confined in the active region. J
th
was reduced to 40kA/cm
2
. 
For charge carrier and optical beam confinement was proposed use of heterostructures. 
A comparison of Figure 
9.
(a) and Figure 
9.
(b) shows that the p+ - GaAs layer of Figure 
9.
(a) has been 
replaced by a p+- Al
x
Ga
1-x
As layer. Since the band-gap of Al
x
Ga
1-x
As is wider than that of p-GaAs, the 
junction between p-GaAs and p+-Al
x
Ga
1-x
As is a single heterojunction, forming SH diode. 
This heterojunction serves two functions: 
1. Confines the optical power in the active layer, which minimizes the optical loss in the cladding 
layer; 
2. Provides a potential barrier at the heterojunction that confines the charge carriers in the active 
region. 

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J
th
decreases to about 10kA/cm
2
. 
Further improvement was achieved by incorporating a second heterojunction, resulting in DH lasers.