Light-emitting diode

A light-emitting diode is a tiny light that turns on when an electric charge is applied to the correct end. It is made of semiconductor.

LEDs used as a bright light source are called high-brightness LEDs (HB LEDs). LEDs can be used as a light source for displays, such as in a backlight. LEDs can be used as the backlights for a liquid-crystal display.

Construction

There is a junction of a p type semiconductor and an n type semiconductor.

Materials/substrates

There is typically an n-type material as the substrate, with p-type deposited on top.

High brightness LEDs are typically made using a sapphire substrate.

The idea is to take a high brightness blue LED and layer phosphor over it. The phosphor turns the blue into white, and does so with a high color renering index (CRI).

Manufacturing

See also: XVRWiki:How to make an LED

LEDs can be made in two steps: the first is making an LED chip, and then the second is packaging the LED into something that can be soldered onto a circuit board.

LED chips

It is recommended to make LEDs using microfabrication processes. LEDs can be made at a large scale using implantation or deposition processes. MOCVD is recommended.

LED chips, including high-brightness LED chips, can be manufactured on wafers.

LED production can be done using a sapphire wafer and doing epitaxial growth on top of it.

It uses epiwafers, meaning epitaxial wafers.

Typically, the wafers are sapphire. The sapphire crystal can be in a number of orientations. It may be typical to have a sapphire substrate aligned to one of the crystal planes: the R plane, C plane, A plane, and M plane. C-plane sapphire substrates can be used to grow gallium nitride (GaN) for making high brightness blue LEDs.

Gallium nitride (GaN) is a good option. A GaN-on-silicon (GaN-on-Si) process can be used, where the base layer is a silicon wafer, and a layer of GaN crystal is grown on top of it.

Wafer size

Machines that support up to 4 inch wafers are probably sufficient for small amounts of manufacturing.

There are various sizes of wafers that can be used, such as 2-inch, 3-inch, and 4-inch.

LEDs may be made on 200mm wafers.

4-inch-wafer LED production was new in the year G2007.

Singulation

LEDs can be singulated using laser scribing. One type of laser singulation is thermal laser singulation, also called laser ablation dicing or thermal ablation. It can be used to dice brittle wafers such as sapphire, LEDs, GaAs, and InP. A laser beam may have limited cut depth, so multiple beam passes of the laser may be needed depending on the wafer thickness and geometry.

Sorting, also known as binning

A sorting process may happen during and/or after a large scale production of LED chips, because some of the LEDs may have slightly different color or brightness once they are made. This is called binning.

Operating principle

Light is produced by the recombination of electrons and electron holes in a semiconductor portion of a light-emitting diode. The color of the light that comes off the chip depends on the band gap of the semiconductors used. These materials have a high index of refraction, so design features such as special optical coatings and die shape are required to efficiently emit light.