Branch : Computer Science and Engineering
Subject : Fundamental of Electronic Devices
Unit : Junction Properties
The Basic Semiconductor Laser
Following section describes the basic semiconductor laser in brief.
The Basic Semiconductor Laser:
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To build a p-n junction laser, we need to form a junction in a highly doped, direct semiconductor (GaAs, for example), construct a resonant cavity in
the proper geometrical relationship to the junction, and make contact to the junction in a mounting which allows for efficient heat transfer.
- The first lasers were built as shown in Fig.
- Beginning with a degenerate n-type sample, a p region is formed on one side, for example by diffusing Zn into the n-type GaAs.
- Since Zn is introduced substitutionally on Ga sites, it serves as an acceptor in GaAs; therefore, the heavily doped Zn diffused layer forms a p region (Fig.b)
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At this point we have a large-area planar p-n junction. Next, grooves are cut or etched, along the length of the sample as in Fig. c, leaving a series of
long p regions isolated from each other.
- These p-n junctions can be cut or broken apart (Fig. d) and then cleaved into devices of the desired length.
- At this point in the fabrication process, the very important requirements of a resonant cavity must be considered.
- It is necessary that the front and back faces (Fig. e) be flat and parallel.
- This can be accomplished by cleaving.
- If the sample has been oriented so that the long junctions of Fig. d are perpendicular to a crystal plane of the material, it is possible to cleave the sample along this plane into laser devices, letting the crystal structure itself provide the parallel faces.
- The device is then mounted on a suitable header, and contact is made to the p region.
- Various techniques are used to provide adequate heat sinking of the device for large forward current levels.