3.2. LIGHT-EMITTING DIODES 87
the laser performance. Suchquantum-well lasershave been studied extensively [14].
Often, multiple active layers of thickness 5–10 nm, separated by transparent barrier
layers of about 10 nm thickness, are used to improve the device performance. Such
lasers are calledmultiquantum-well(MQW) lasers. Another feature that has improved
the performance of MQW lasers is the introduction of intentional, but controlled strain
within active layers. The use of thin active layers permits a slight mismatch between
lattice constants without introducing defects. The resulting strain changes the band
structure and improves the laser performance [5]. Such semiconductor lasers are called
strainedMQW lasers. The concept of quantum-well lasers has also been extended to
make quantum-wire and quantum-dot lasers in which electrons are confined in more
than one dimension [14]. However, such devices were at the research stage in 2001.
Most semiconductor lasers deployed in lightwave systems use the MQW design.
3.2 Light-Emitting Diodes
A forward-biasedp–njunction emits light through spontaneous emission, a pheno-
menon referred to as electroluminescence. In its simplest form, an LED is a forward-
biasedp–nhomojunction. Radiative recombination of electron–hole pairs in the deple-
tion region generates light; some of it escapes from the device and can be coupled into
an optical fiber. The emitted light is incoherent with a relatively wide spectral width
(30–60 nm) and a relatively large angular spread. In this section we discuss the char-
acteristics and the design of LEDs from the standpoint of their application in optical
communication systems [20].
3.2.1 Power–Current Characteristics.................
It is easy to estimate the internal power generated by spontaneous emission. At a given
currentIthe carrier-injection rate isI/q. In the steady state, the rate of electron–hole
pairs recombining through radiative and nonradiative processes is equal to the carrier-
injection rateI/q. Since the internal quantum efficiencyηintdetermines the fraction of
electron–hole pairs that recombine through spontaneous emission, the rate of photon
generation is simplyηintI/q. The internal optical power is thus given by
Pint=ηint(h ̄ω/q)I, (3.2.1)where ̄hωis the photon energy, assumed to be nearly the same for all photons. Ifηext
is the fraction of photons escaping from the device, the emitted power is given by
Pe=ηextPint=ηextηint(h ̄ω/q)I. (3.2.2)The quantityηextis called theexternal quantum efficiency. It can be calculated by
taking into account internal absorption and the total internal reflection at the semicon-
ductor–air interface. As seen in Fig. 3.6, only light emitted within a cone of angle
θc, whereθc=sin−^1 ( 1 /n)is the critical angle andnis the refractive index of the
semiconductor material, escapes from the LED surface. Internal absorption can be
avoided by using heterostructure LEDs in which the cladding layers surrounding the