Optoelectronic Semiconductor Devices Principals and Characteristics

Figure 5.: Emission wavelengths of selected semiconductor ILDs. Solid and dashed lines represent 
continuous operation at room temperature and cryogenic temperature. 
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For a thin active region, one can move slightly left or right of the lattice matching condition, as it was 
mentioned above. In this case, the lattice of the deposited film distorts so as to fit the substrate lattice in 
the plane, but it also must distort in the perpendicular direction to retain approximately the same unit cell 
volume it would have without distortion. 
 
Figure 6.: Schematic of sandwiching quantum wells with either a larger or smaller lattice constant to 
provide either compressive or tensile strain, respectively.
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Figure 
6.
 shows a cross section of how unit cells might distort to accommodate a small lattice mismatch. 
It turns out that such strained layers may have more desirable optoelectronic properties than their 
unstrained counterparts. Plus, quantum-well active regions, which are thinner than typical critical 
thicknesses, are desirable in diode lasers for reduced threshold and improved thermal properties. 
2.3 BASIC SEMICONDUCTOR LUMINESCENT DIODE 
STRUCTURES 
The heart of the semiconductor luminescent diode is an active semiconducting layer, which is sandwiched 
between two cladding layers. There are two junctions, one on each side of the active layer. 
In order for the gain material in a semiconductor laser to function, it must be pumped or excited with 
some external energy source. A major attribute of diode lasers is their ability to be pumped directly with 
an electrical current. Of course, the active material can also be exited by the carriers generated from 
absorbed light, and this process is important in characterizing semiconductor material before electrical 
contacts are made. 
The first ILDs and LEDs were homojunction diodes. Most modern electro luminescent diodes have one 
ore two heterojunctions. 
SH - single heterostructure diode: has two different materials and has both a homojunction and 
heterojunction. 
DH - double heterostructure diode: formed with three materials and two heterojunctions. 
After many early efforts that used homojunctions or single heterostructure, the advent of the DH structure 
made the diode laser truly practical for the first time. 
To form the necessary resonant cavity for optical feedback, simple cleaved facets can be used, since the 
large index of refraction discontinuity at the semiconductor-air interface provides a reflection coefficient 
of ~30%. The lower band-gap active region also usually has a higher index of refraction, n, than the 
cladding. 

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(a) a schematic of the material structure; 
(b) an energy diagram of the conduction and valence bands vs. transverse distance; 
(c) the refractive index profile; 
(d) the electric field profile for a mode traveling in the z-direction. 
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