Elements of Modern Physics
290
in Ge and Si, radiative recombinations are important in some semi-conductors
such as GaAs. In GaAs the energy gap is about 1.4 eV corresponding to a
frequency for λ ≈ 8900 Å which is in the infrared region. The gap can be increased
by alloying the material with phosphorus, i.e., by using Ga (As) 1 – x P x which
can produce radiation in the optical region. This radiation can be extracted from
openings close to the junction. Light emitting diodes are used in display and
warning devices.
Semiconductor Diode Laser
If a pn junction is designed properly, a light emitting diode can produce laser
action.
In a semiconductor laser, the laser medium is usually a pn junction of a
semiconductor such as Ga(As) 1 – x P x in which radiative recombinations dominate.
The opposite surfaces of the crystal, perpendicular to the junction, are taken
along the cleavage planes, and are polished so as to form an effective resonance
cavity. In this cavity, the photons emitted at the junction and moving in a
particular direction parallel to the junction plane, produce stimulated emission
giving rise to laser action (Fig. 8.14).
n
T
Junction plane
Laser beam
p
Polished surface
Fig. 8.14 Schematic diagram of a semiconductor laser.
The advantages of semiconductor lasers are that they are compact, efficient
and can be fabricated with ease. However, their monochromaticity, coherence
and directionality are inferior to those of other lasers.
Fabrication of Semiconductor Devices
The working of semiconductor devices depends on the ordered periodic
arrangement of atoms in a lattice and the introduction of impurities in a controlled
manner. A highly sophisticated technology is involved in their development.
The steps involved in their production are illustrated by considering the specific
case of a silicon npn junction.
In the first step, a silicon crystal is grown by dipping a seed crystal into the
molten silicon at about 1425°C, and slowly pulling it up at a rate of about 100
millimetres per hour. The melt contains a suitable amount of phosphorus to
produce new crystal layers of the n-type. The crystal is cut into thin slices about
0.25–0.5 mm in thickness and about 10 cm in diameter, by using a diamond
290
in Ge and Si, radiative recombinations are important in some semi-conductors
such as GaAs. In GaAs the energy gap is about 1.4 eV corresponding to a
frequency for λ ≈ 8900 Å which is in the infrared region. The gap can be increased
by alloying the material with phosphorus, i.e., by using Ga (As) 1 – x P x which
can produce radiation in the optical region. This radiation can be extracted from
openings close to the junction. Light emitting diodes are used in display and
warning devices.
Semiconductor Diode Laser
If a pn junction is designed properly, a light emitting diode can produce laser
action.
In a semiconductor laser, the laser medium is usually a pn junction of a
semiconductor such as Ga(As) 1 – x P x in which radiative recombinations dominate.
The opposite surfaces of the crystal, perpendicular to the junction, are taken
along the cleavage planes, and are polished so as to form an effective resonance
cavity. In this cavity, the photons emitted at the junction and moving in a
particular direction parallel to the junction plane, produce stimulated emission
giving rise to laser action (Fig. 8.14).
n
T
Junction plane
Laser beam
p
Polished surface
Fig. 8.14 Schematic diagram of a semiconductor laser.
The advantages of semiconductor lasers are that they are compact, efficient
and can be fabricated with ease. However, their monochromaticity, coherence
and directionality are inferior to those of other lasers.
Fabrication of Semiconductor Devices
The working of semiconductor devices depends on the ordered periodic
arrangement of atoms in a lattice and the introduction of impurities in a controlled
manner. A highly sophisticated technology is involved in their development.
The steps involved in their production are illustrated by considering the specific
case of a silicon npn junction.
In the first step, a silicon crystal is grown by dipping a seed crystal into the
molten silicon at about 1425°C, and slowly pulling it up at a rate of about 100
millimetres per hour. The melt contains a suitable amount of phosphorus to
produce new crystal layers of the n-type. The crystal is cut into thin slices about
0.25–0.5 mm in thickness and about 10 cm in diameter, by using a diamond