Optoelectronic Semiconductor Devices Principals and Characteristics

 
Figure 7.: Aspects of the DH diode laser:
Figure 
7.
gives a schematic of a broad-area pin DH laser diode, along with transverse sketches of the 
energy gap, index of refraction and resulting optical mode profile across the DH region. A thin slab of 
undoped active material is sandwiched between p and n-type cladding layers, which have a higher 
conduction-valence band energy gap. Typical thickness of the active layer for this simple three-layer 
structure are ~0.1-0.2 µm. Because the band-gap of the cladding layers is larger, light generated in the 
active region will not have sufficient photon energy to be absorbed in them, i.e. E
21
=hν<E
gcl
. 
For this DH structure, a transverse (x-direction) potential well is formed for electrons and holes that are 
being injected from the n- and p-type regions, respectively, under forward bias. They are captured and 
confined together, thereby increasing their probability of recombining with each other. 
Unlike in most semiconductor diodes or transistors that are to be used in purely electronic circuits, it is 
desirable to have all of the injected carriers recombine in the active region to form photons in a laser or 
LED. Unfortunately, there is always a possibility of the ''leakage current'', which results from some of the 
carriers being thermionically emitted over the heterobarriers before they can recombine. 
One should also realize that if the end facet reflections are suppressed by antireflection coating, the device 
would then function as an LED. 

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Many modern diode lasers involve a little more complexity in their transverse carrier and photon 
confinement structure as compared on Figure 
7.
, but the fundamental concepts remain valid. 
Figure 
8.
 illustrates transverse band-gap profiles for such separate-confinement heterostructure, single 
quantum-well (SCH-SQW) lasers. The transverse optical energy density is also overlaid to show that the 
photons are confined primarily by the outer heterointerfaces and the carriers by the inner quantum well. 
(a) standard SCH; 
(b) Graded-index SCH (GRINSCH). 
The electric field (photons) are confined by the outer step or graded heterostructure; the central quantum 
well confines the electrons. 
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