156
4 Light Sources for Fiber Links
the barrier potential is reduced. Thereby conduction-band electrons on the n side and
valence-band holes on the p side are allowed to diffuse across the junction. Once
across, they significantly increase the minority carrier concentrations, and the excess
carriers then recombine with the oppositely charged majority carriers. The recombination of excess minority carriers is the mechanism by which optical radiation is
generated.
4.1.4 Direct Bandgap and Indirect Bandgap
As Sect. 2.1.4 and Eq. (4.3) note, a photon has an energy E = hν = hc/λ, where ν
and λ are the frequency and wavelength, respectively, that are associated with the
photon. When electron transitions occur between the valence and conduction bands,
an electron in the valence band can absorb the energy from an impinging photon
and thereby get boosted to the conduction band. In the case when an electron drops
from the conduction band and combines with a hole in the valence band, a photon
gets emitted in this process as Fig. 4.7 illustrates. In order for electron transitions
to take place to or from the conduction band, both energy and momentum must be
conserved. Although a photon can have considerable energy, its momentum hv/c is
very small.
Semiconductors are classified as either direct-bandgap or indirect-bandgap materials depending on the shape of the bandgap as a function of the momentum k, as
shown in Fig. 4.7. Now consider the recombination of an electron and a hole, accompanied by the emission of a photon. The simplest and most probable recombination
process will be that where the electron and hole have the same momentum value (see
Fig. 4.7a). This is a direct-bandgap material.
For indirect-bandgap materials, the conduction band minimum and the valence
band maximum energy levels occur at different values of momentum, as shown in
Fig. 4.7b. Here, band-to-band recombination must involve a third particle to conserve
momentum because the photon momentum is very small. Phonons (i.e., crystal lattice
vibrations) serve this purpose.
4.1.5 Fabrication of Semiconductor Devices
In fabricating semiconductor devices, the crystal structure of the various material
regions must be carefully taken into account. In any crystal structure, single atoms
(e.g., Si or Ge) or groups of atoms (e.g., NaCl or GaAs) are arranged in a repeated
pattern in space. This periodic arrangement defines a lattice, and the spacing between
the atoms or groups of atoms is called the lattice spacing or the lattice constant.
Typical lattice spacings are a few angstroms (1 angstrom = 0.1 nm).
Semiconductor devices generally are fabricated by starting with a crystalline
substrate that provides mechanical strength for mounting the device and for making
4 Light Sources for Fiber Links
the barrier potential is reduced. Thereby conduction-band electrons on the n side and
valence-band holes on the p side are allowed to diffuse across the junction. Once
across, they significantly increase the minority carrier concentrations, and the excess
carriers then recombine with the oppositely charged majority carriers. The recombination of excess minority carriers is the mechanism by which optical radiation is
generated.
4.1.4 Direct Bandgap and Indirect Bandgap
As Sect. 2.1.4 and Eq. (4.3) note, a photon has an energy E = hν = hc/λ, where ν
and λ are the frequency and wavelength, respectively, that are associated with the
photon. When electron transitions occur between the valence and conduction bands,
an electron in the valence band can absorb the energy from an impinging photon
and thereby get boosted to the conduction band. In the case when an electron drops
from the conduction band and combines with a hole in the valence band, a photon
gets emitted in this process as Fig. 4.7 illustrates. In order for electron transitions
to take place to or from the conduction band, both energy and momentum must be
conserved. Although a photon can have considerable energy, its momentum hv/c is
very small.
Semiconductors are classified as either direct-bandgap or indirect-bandgap materials depending on the shape of the bandgap as a function of the momentum k, as
shown in Fig. 4.7. Now consider the recombination of an electron and a hole, accompanied by the emission of a photon. The simplest and most probable recombination
process will be that where the electron and hole have the same momentum value (see
Fig. 4.7a). This is a direct-bandgap material.
For indirect-bandgap materials, the conduction band minimum and the valence
band maximum energy levels occur at different values of momentum, as shown in
Fig. 4.7b. Here, band-to-band recombination must involve a third particle to conserve
momentum because the photon momentum is very small. Phonons (i.e., crystal lattice
vibrations) serve this purpose.
4.1.5 Fabrication of Semiconductor Devices
In fabricating semiconductor devices, the crystal structure of the various material
regions must be carefully taken into account. In any crystal structure, single atoms
(e.g., Si or Ge) or groups of atoms (e.g., NaCl or GaAs) are arranged in a repeated
pattern in space. This periodic arrangement defines a lattice, and the spacing between
the atoms or groups of atoms is called the lattice spacing or the lattice constant.
Typical lattice spacings are a few angstroms (1 angstrom = 0.1 nm).
Semiconductor devices generally are fabricated by starting with a crystalline
substrate that provides mechanical strength for mounting the device and for making