LED Structure & Fabrication

- summary of different LED structure types and the fabrication methods used.

LEDs are a specialised form of p-n junction diode that have been designed to optimise their electroluminescence. As a result the LED structure and LED fabrication techniques need 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

There are two basic configurations for the LED structure.

  • Surface emitting LED structure:   This form of LED structure emits light perpendicular to the plane of the PN junction.
  • 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.

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.

Final LED package structure

There are obviously many different styles of LED that are available. These range from the simple LED indicators through the more complicated LED alphanumeric displays to the LED screens that are now appearing. All these types of LED will have their own package structure. However the simple LED indicators tend to have a fairly common structure and this serves to indicate the constraints on any LED device.

The structure of the LED package can be split into a number of different elements:

  • Semiconductor die :  This is the light emitting diode itself formed from the semiconductor.
  • Lead frame:  This houses the die and acts as the connection to it.
  • Encapsulation:  This surrounds the assembly and acts as protection as well as dispersing the light.

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.

Typical LED package
Typical LED package

By Ian Poole

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