This is Part 2 of a 4 Part Series
- Surface Mount LEDs – Part 1: The Basics
- Surface Mount LEDs – Part 2: How They are Made (this article)
- Surface Mount LEDs – Part 3: Optical Properties
- Surface Mount LEDs – Part 4: Applications
Solid state Light Emitting Diodes (LEDs) are produced using thin film (TF) technology. Additionally, surface mount technology (SMT) is used to package the solid state lighting element into a useful form that exhibits desirable optical and other properties. These cornerstone technologies are robust tools that are used with ubiquity throughout the electronics industry.
In order to make the light emitting element of the LED, very pure materials are fabricated in single crystal form. This is typically done either by growing a single crystal ingot from a Bridgeman-grown boule or the like using a form of the Czochralski method, or by forming very thin single crystal layers on a substrate, using thin film techniques such as heteroepitaxial deposition or the like. The highly pure single crystal LED elements may then be further purified using a zone refining method or the like.
The light emitting elements are then made into semiconductors, generally by doping them with one or more of an n-type or a p-type dopant material. This is achieved by way of a highly controlled process such as ion implantation or thermal diffusion. Doping is valuable not only in establishing the semiconducting band structure (an example is illustrated below), but also in establishing a p-n junction which creates the diode (i.e., bias controlled switch). Next damage from dislocations or similar, that are formed during the doping process, are addressed by way of a rapid thermal annealing (RTA) process or the like.
The dopant materials are carefully selected for their ability to either donate or accept electrons, as well as for ionic size compatible with the structure of the single crystal host material. These controlled addition(s) and location(s) of the proper dopant chemistry effectively reduce the band gap of the single crystal semiconductor, and also serve to provide a charge transport means (e.g., excess electrons in the conduction band and excess electron holes in the valence band) as illustrated above.
With the proper amount of forward bias, electrons will “jump” the engineered bandgap at the p-n junction, thus flowing current. In the case of LEDs, light of a specific wavelength is emitted when the electrons in the conduction band recombine with the electron holes in the valence band. Higher energy emitted light requires a larger energy bandgap, and thus a higher forward bias in order to function.
Some typical single crystal materials used for each LED color regime are illustrated below. For example, either Aluminum-Gallium-Nitride or Indium-Gallium-Nitride may be used for the shorter wavelengths (blues, etc.), while either Gallium-Phosphide or Aluminum-Gallium-Phosphide may be used for longer wavelength (reds, etc.). The dopant treatment is selected to further engineer the bandgap, and thus the refine the color. The available spectral range of LEDs is ever increasing as new materials and dopant schemes are discovered, extending the range well into the infrared as well as into the UV portions of the electromagnetic spectrum.
After the light emitting elements are fabricated they need to be packaged into a useful form, having useful properties. As the indicator light market has evolved, it has come to favor SMT packaging. An example of the SMT packaging production process is illustrated below.
The finished LED wafers are first tested in a die sorter in order to ensure quality. The singulated individual LED die are then bonded to a conductive leadframe using a pick and place die bonder, and then the corresponding bonding adhesive is cured. The I/O pads for each die are then electrically connected to the leadframe typically using wire bonding. A phosphor (if needed for frequency shifting or blending or other optical properties) is then applied. The mounted assembly is then encapsulated, and the lens material (or the lens), etc. is applied or molded over the LED die on the portions of the package from which light emission is desired. The SMT LED packages are then singulated from the multi-up leadframe, and are then tested, sorted and packaged for shipping and for assembly at the customer.
Other packing schemes are also possible; for example those resulting in LED displays, etc. These schema enable use of LEDs in myriad applications, ranging from high resolution flexible displays (OLEDs) to high brightness automotive headlamps (HBLEDs) and everywhere in between. The impressive developments in advanced semiconductor materials and related technologies have resulted in advanced LEDs that are now the “winning” option for nearly all lighting applications.