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Lasers 125

Dielectric reflective p-type GaAs – active region
layer Laser output

p-type AlGaAs

n-type AlGaAs

Cleaved facet

Figure 3.33 A double-heterojunction Fabry–Perot laser diode. The cleaved end functions as a partially reflecting mirror.

Active region

p-type n-type

(a)
Heterojunctions

Conduction band
Energy E g1 E g2 E g1

Valence band
(b)

Index, n

n 2
n 1 n 1
(c)

Figure 3.34 Double heterostructure: (a) heterojunctions; (b) band gap; (c) refractive index.

for holes in the valence band. Electrons and holes trapped in the active region could escape to the surrounding
layers only if they have sufficient energy to cross the barriers. As a result, both electrons and holes are mostly
confined to the active region. Because of the smaller band gap, the active region has a slightly higher refractive
index. As discussed in Chapter 2, this acts as an optical waveguide and light is confined to the middle layer of
the higher refractive index due to total internal reflection. Therefore, not only electrons and holes are confined