Other diodes: Diode types
There may appear to be many similarities between a light emitting diode and a laser diode, the two are fundamentally different from an operational point of view.
The laser diode is consists of heavily doped n+ and p+ regions. For manufacture it is normal to start with an n+ substrate and then the top layer can be grown onto this.
The doping can be included in a variety of ways, either by diffusion, ion implantation or even deposited during the epitaxy process. A variety of materials can be used for laser diodes, although the most common starting substrates are Gallium Arsenide (GaAs) and Indium Phosphate (InP).
These are known as type III-V compounds because of their places in the chemical periodic table of elements. Whatever material is used, it must be possible to heavily dope it as either a p type or n type semiconductor. This rules out most of the type II-VI materials, leaving the group III-V materials as the ideal option.
Apart from the basic semiconductor requirements, there are a number of optical requirements that are needed to enable the laser diode to operate. It needs an optical resonator. This must occur in the plane of the required light output.
To achieve this the two walls of the laser diode that form the resonator must be almost perfectly smooth, forming a mirror surface from which the light can be reflected internally. One of the walls is made slightly less reflecting to enable the light to come out from the laser diode.
Another requirement is that the two mirror surfaces must be perfectly perpendicular to the junction, otherwise the laser action does not occur satisfactorily. The two other surfaces perpendicular to the one of the required light output are roughened slightly to ensure that the laser action does not occur in this plane as well. In this way a resonant optical cavity is created. Although it is many wavelengths long it still acts as a resonant cavity.
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