3.1. BASIC CONCEPTS 83
Figure 3.4: Simultaneous confinement of charge carriers and optical field in a double-
heterostructure design. The active layer has a lower bandgap and a higher refractive index than
those ofp-type andn-type cladding layers.
3.1.3 Nonradiative Recombination
When ap–njunction is forward-biased, electrons and holes are injected into the ac-
tive region, where they recombine to produce light. In any semiconductor, electrons
and holes can also recombine nonradiatively. Nonradiative recombination mechanisms
include recombination at traps or defects, surface recombination, and the Auger recom-
bination [5]. The last mechanism is especially important for semiconductor lasers emit-
ting light in the wavelength range 1.3–1.6μm because of a relatively small bandgap
of the active layer [2]. In the Auger recombination process, the energy released dur-
ing electron–hole recombination is given to another electron or hole as kinetic energy
rather than producing light.
From the standpoint of device operation, all nonradiative processes are harmful, as
they reduce the number of electron–hole pairs that emit light. Their effect is quantified
through theinternal quantum efficiency, defined as
ηint=Rrr
Rtot=
Rrr
Rrr+Rnr, (3.1.16)
whereRrris the radiative recombination rate,Rnris the nonradiative recombination