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There are very many different types of LED, but to explain how they are made and what their structure is, the best starting point is the structure and fabrication of a simple indicator light emitting diode.
Light emitting diodes are a specialised form of PN junction diode that have been designed to optimise their electroluminescence. To achieve the optimum results, the structure and fabrication processes need to be tailored to ensure that the light output is optimised.
There are a number of different aspects to the LED structure and LED fabrication. These include not only the LED fabrication itself, but also the packaging of the LED once the semiconductor chip itself has been fabricated.
LED die structure
The die or the actual semiconductor element of the light emitting diode is the active part of the overall diode. There are two basic configurations which can be used.
- Edge emitting LED structure: This form of LED structure emits light in a plane parallel to the junction of the PN junction. In this configuration the light can be confined to a narrow angle.
- Surface emitting LED structure: This form of LED structure emits light perpendicular to the plane of the PN junction.
The active films of the LED structure are normally grown epitaxially - often by liquid phase or vapour phase epitaxy. The substrates are chosen to have a close lattice match to the active layers.
Common substrates are GaAS, GaP, InP. The PN junction can be created by either impurity diffusion, ion implantation, or it can be incorporated during the epitaxial growth phase.
Commercially, LEDs exist in a variety of forms, ranging from individual LED indicators where there is just one LED per package, through a variety of displays, right up to vast arrays of LEDs in LED screens.
For some limited applications, it is possible to use a variety of LED PN diode junction types. These can include Schottky contacts and MIS (metal-intrinsic-semiconductor) junctions. However these are normally less efficient and sometimes more difficult to form reliably.
Overall LED package structure
There are many different styles of LED that are available. One of the most widely used is the simple LED indicator package. This provides insight intot he way in which LEDs can be packaged.
In addition to the very simple LED indicator, there are many other types of LED including alphanumeric displays, and more complicated LED displays.
The structure of the panel indicator LED package can be split into three main elements:
- Semiconductor die: This is the light emitting diode semiconductor element itself.
- Lead frame: This portion of the LED package houses the die and provides the connection.
- Encapsulation: This surrounds the assembly and acts as protection. It is also designed to disperse the light in the required manner.
The die is bonded into a recess in one half of the lead frame, called the anvil due to its shape. This is done using conductive epoxy. The recess in the anvil is shaped to throw the light radiation forward. The top contact from the die is then wire-bonded to the other lead frame terminal which is often called the post.