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Excess holes
Excess electrons
Fermi Level
Fermi Level
Fermi Level
Excess holes
Excess electrons
They are basically a diode
junction. In order to get LASER
action there needs to be a
region where BOTH excited
electron states and holes
(vacant electron states) are
This is achieved using heavily
doped n and p material and
applying a forward bias to the
junction. Fig. 16 shows the
energy levels either side of the
diode junction.
Diode Pumped Solid State (DPSS) laser: Diode lasers make a
convenient pump source for various gain media, typically those
solid state media utilizing Neodymium as the dopant
(Neodymium:Yttrium Aluminum Garnet or Nd:YAG, Nd:Yttrium
Vanadate or Nd:YVO4). The 1064 nm output is typically frequency
doubled to 532nm. The familiar green laser pointer is a DPSS
Figure 3.2 Indirect and Direct gap semiconductors
In equilibrium, the charge carriers occupy their lowest energy states, electrons at the bottom of the conduction band, and
holes at the top of the valence band. In silicon these states do not have the same momentum. Therefore if a recombination is
to result in the emission of a photon, which has little momentum, a quantum of lattice vibration (a phonon) must also be
created to carry away the excess momentum. This is known as an indirect process and such semiconductors are known as
INDIRECT GAP semiconductors. The two particle process is not favoured, and recombinations in indirect gap
semiconductors usually occur by thermal or collisional processes. The silicon chips in a computer do not glow, they just get
GaAs however is a DIRECT GAP semiconductor. The minima of electron and hole energies occur at the same momentum.
Thus recombination can result in a photon alone. LED action was first observed in GaAs in 1952. The light is emitted in a
narrow range of wavelengths which is determined by the size of the band gap of the semiconductor. The red, green and
yellow LEDs available today are made using semiconductor compounds with different band gaps, for example GaAs,
GaAlAs, AlInGaP, GaAsP.
The LED must satisfy two more conditions before it is possible to make a laser diode. Firstly stimulated emission must be
able to dominate over absorbtion and spontaneous emission, and secondly there must be an optical cavity. Fortunately both
are relatively simple to achieve. The population inversion required for stimulated emission to dominate is realised by
increasing the current through the diode. This increases the density of electron-hole pairs in the junction region, and thus
creates an inversion. The optical cavity is made by cleaving the crystal along two parallel crystal planes, perpendicular to the
junction plane. This creates optically flat and parallel surfaces. They do not need to be coated to act as laser mirrors as the
high refractive index of GaAs gives the GaAs-air interface a reflectivity of 35% which is found to give sufficient feedback to
sustain laser action.
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