82 CHAPTER 3. OPTICAL TRANSMITTERS
(a) (b)Figure 3.3: Energy-band diagram of (a) homostructure and (b) double-heterostructurep–njunc-
tions in thermal equilibrium (top) and under forward bias (bottom).
not be doped, depending on the device design; its role is to confine the carriers injected
inside it under forward bias. The carrier confinement occurs as a result of bandgap
discontinuity at the junction between two semiconductors which have the same crys-
talline structure (the same lattice constant) but different bandgaps. Such junctions are
calledheterojunctions, and such devices are calleddouble heterostructures. Since the
thickness of the sandwiched layer can be controlled externally (typically,∼0.1μm),
high carrier densities can be realized at a given injection current. Figure 3.3(b) shows
the energy-band diagram of a double heterostructure with and without forward bias.
The use of a heterostructure geometry for semiconductor optical sources is doubly
beneficial. As already mentioned, the bandgap difference between the two semicon-
ductors helps to confine electrons and holes to the middle layer, also called the active
layer since light is generated inside it as a result of electron–hole recombination. How-
ever, the active layer also has a slightly larger refractive index than the surrounding
p-type andn-type cladding layers simply because its bandgap is smaller. As a result
of the refractive-index difference, the active layer acts as a dielectric waveguide and
supports optical modes whose number can be controlled by changing the active-layer
thickness (similar to the modes supported by a fiber core). The main point is that a
heterostructure confines the generated light to the active layer because of its higher
refractive index. Figure 3.4 illustrates schematically the simultaneous confinement of
charge carriers and the optical field to the active region through a heterostructure de-
sign. It is this feature that has made semiconductor lasers practical for a wide variety
of applications.