Optoelectronic Semiconductor Devices Principals and Characteristics

 
Figure 10.: Schematic representation of energy band diagram, refractive index profile, and optical field 
distribution of double heterostructure LEDs and ILDs. 
[1]
Figure 
10.
 shows the schematic of the DH diode. 
Difference with the SH diodes: 
- potential barrier for holes at pN junction and for electrons at the Pp junction; 
- the index difference between the active layer and surrounding cladding layers also acts as a guide 
for the resulting optical waves. 
Because of these improvements, J
th
can be reduced further to 0.5kA/cm
2
at the room temperature. 
Further reduction of the threshold current density is possible by using very thin layers which are known 
as quantum wells (In the thin layer, carrier motion perpendicular to the layer is restricted, and the kinetic 
energy is quantized into discrete energy levels. Structures consisting of such extremely thin layers having 
quantized energy levels are called quantum well structures), the thin layer is called potential well, and one 
of the neighboring layers with wide band-gap energies is called a potential barrier. One of the key results 

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of the potential well is that electrons in solids can behave much like free electrons with plane wave 
solutions and a parabolic E-k relationship. 
2.4 SUMMARY ON THE BASIC STRUCTURE OF MODERN 
ILD, LED AND SLD 
All modern ILDs, LEDs and SLDs consist of an active layer and two cladding layers, surrounded by a 
substrate on one side and a contact layer on the other side. (Figure 
11.
). 
• 
[1]The active layer is the light generating medium of the diode. It should be a direct band-gap 
material, with E
g
corresponding to the desired emission wavelength. The band-gap should be 
narrower and the refractive index grater than those of the surrounding cladding regions. 
• 
[2]The cladding layers restrict the motion of the charge carriers and confine the optical power in 
the active layer. The band-gap must be wider than band-gap of the active layer in order to prevent 
the charge carriers from diffusing away from the active layer. Index of refraction must be lower 
than that of the active layer in order to confine the optical power in the active region. Since no 
photons are emitted from the cladding layers, materials used can have direct or indirect band-gap. 
• 
[3]The substrate provides mechanical support, heat dissipation capability and electrical contact. 
Proper crystal symmetry and lattice constant of the substrate are necessary for the growth of the 
succeeding compound semiconductor layers or solid solutions. 
• 
[4]The contact layer provides electrical contact. In the case of surface emitting diodes, this layer 
should also be transparent to the emission wavelength.