Colored LEDs: how do you get the different colors?

Colored LEDs

The Colored LEDs They have been accompanying us in recent years. Every time new shades of LEDs appear, since it has not been easy in all cases. For example, as a curiosity, you should know that white light LEDs and blue light LEDs have been among the last to arrive on the market.

Currently, they have become a type of diode essential for many fields. Therefore, in this article you will learn All you need to know On these basic electronic components, and about why they emit light, why those colors, and much more...

Semiconductor Light Emitting Sources

LED diode

As you should know, the two sources of light emission that can come from semiconductor devices are Laser diodes and LED diodes. While LED is based on spontaneous emission, Lasers are based on stimulated emission. That is the difference between the two.

The light emitting diodes (Light Emitting Diode) they are the most common light source among electronic equipment. They are used to show the time on digital watches, to signal the operation or charge of the battery, etc. The applications are many, and now they have also jumped into lighting with the new LED bulbs to illuminate all types of rooms and even for vehicles.

These LED devices belong to the group of opto-semiconductors, capable of converting an electric current into light. This lighting device has the great advantage of being durable, since it does not burn out like light bulbs, and it is also much more efficient, so consumption is much lower than conventional light bulbs. In addition, their manufacturing cost is very low, which is why they have become so popular.

Like any other semiconductor device, the LED has the basic main elements, such as the P zones with holes (+) and N zones with electrons (-), that is, the usual charge carriers of any semiconductor. And this makes:

  • When the P side is connected to a power supply and the N side to ground, the connection is forward biased, allowing current to flow through the diode and emitting light that we can all see.
  • If the P side is connected to ground and the N side is connected to the power supply, the connection is said to be reverse biased, which prevents the flow of current. You already know that diodes prevent the flow of current in one direction.
  • When forward biased, the P-side and N-side majority and minority charge carriers combine with each other, neutralizing charge carriers in the depletion layer of the PN junction. And, in turn, this migration of electrons and holes releases a certain amount of photons, that is, part of the energy is emitted in the form of light, with a constant (monochromatic) wavelength. This is what will characterize the color of the LED, since depending on the wavelength it emits it can be IR, blue, yellow, green, yellow, amber, white, red, UV, etc.
  • The emitted wavelength of the electromagnetic spectrum, and therefore the color, is determined by the semiconductor materials that form the PN junction of the diode. Therefore, semiconductor compounds can be varied or played with to create new colors within the spectrum or visible range.

It must be said that the colors red, blue and green (RGB or Red Green Blue) can be easily combined to be able to produce white light. On the other hand, it must be said that the working voltage of the LEDs also varies depending on the color. For example, the colors red, green, amber, and yellow need about 1.8 volts to work. And it is that the working voltage range of the light emitting diode can be determined according to the breakdown voltage of the semiconductor material used for the manufacture of the LED.

LED types

laser diode

LEDs can be classified in several ways, one of the main ones is to do it according to the wavelength they emit, leaving two categories:

  •  visible LEDs: are those that emit wavelengths within the visible spectrum, that is, between 400nm and 750nm. This range is what the human eye can see, just as in the sound field we can only hear between 20 Hz and 20 Khz. Below 20 Hz are infrasound that we cannot hear, and above 20 Khz are ultrasound that we cannot capture either. Something similar happens in the case of light, having infrared or IR when it goes below 400 nm and ultraviolet light when it goes above 750 nm. Both invisible to the human eye.
  •  invisible LEDs: are those wavelengths that we cannot see, as is the case with an IR diode or a UV diode.

Visible LEDs are mainly used for lighting or signaling. Invisible LEDs are used in applications including optical switches, optical communications and analysis, etc., with the use of photo sensors.

Efficiency

As you well know, LED lighting is much more efficient than conventional, so it consumes much less energy. This is due to the nature of LEDs. And in the following table you can see the relationship between the luminous flux and the electrical input power supplied to the LED. That is, it can be expressed in lumens per watt (lm/W):

Color
Wavelength (nm)
Typical efficacy (lm/W)
Typical Efficiency (w/w)
RED
620 – 645
72
0.39
Verde
520 – 550
93
0.15
BLUE
460 – 490
37
0.35
Cyan
490 – 520
75
0.26
Orange
610 – 620
98
0.29

LED Construction

Manufacture of an LED

Source: ResearchGate

La structure and construction of light emitting diodes are very different from those of a normal diode, such as a zener, etc. Light will be emitted from the LED when its PN junction is forward biased. The PN junction is covered by a solid epoxy resin and transparent plastic hemispherical dome that protects the interior of the LED from atmospheric disturbances, vibrations and thermal shocks.

The PN junction is formed using the materials lower bandgap compounds such as gallium arsenide, gallium arsenide phosphide, gallium phosphide, indium gallium nitride, gallium aluminum nitride, silicon carbide, etc. For example, red LEDs are built on gallium arsenide substrate, green, yellow and orange on gallium phosphide, etc. In the reds, the N-type layer is doped with tellurium (Te) and the P layer is doped with zinc (Zn). On the other hand, the contact layers are formed using aluminum on the P side and tin-aluminum on the N side.

Also, you should know that these junctions do not emit a lot of light, so the epoxy resin dome it is constructed in such a way that photons of light emitted by the PN junction are best reflected and focused through it. That is, it not only acts as a protector, but also as a light concentrating lens. It is the reason why the emitted light appears to be brighter at the top of the LED.

The LEDs are designed to ensure that the most of the recombination of charge carriers takes place at the surface of the PN junction for obvious reasons, and that is achieved in this way:

  • By increasing the doping concentration of the substrate, additional minority charge carrier electrons move to the top of the structure, recombine, and emit light on the LED surface.
  • By increasing the diffusion length of the charge carriers, that is, L = √ Dτ, where D is the diffusion coefficient and τ is the lifetime of the charge carrier. When it is increased beyond the critical value, there will be a possibility of reabsorption of the released photons in the device.

Thus, when the LED diode is connected with forward bias, cargo carriers they acquire enough energy to overcome the existing potential barrier at the PN junction. Minority charge carriers in both the P-type and N-type semiconductor are injected across the junction and recombine with the majority carriers. The combination of majority and minority carriers can be in two ways:

  • radiative: when light is emitted during recombination.
  • not radiative: during recombination no light is emitted, heat is produced. That is, part of the electrical energy applied is lost in the form of heat and not light. Depending on the percentage of energy used to generate light or heat, this will be the efficiency of the LED.

organic semiconductors

Recently they have also broken into the market OLED or organic light-emitting diodes, which have been used for displays. These new organic diodes are composed of a material of organic nature, that is, an organic semiconductor, where conduction is allowed in part or in all of the organic molecule.

These organic materials may be in crystalline phase or in polymeric molecules. This has the advantage of having a very thin structure, low cost, they need very low voltage to operate, they have high brightness, and the maximum contrast and intensity.

LED colors

Colored LEDs

Unlike normal semiconductor diodes, LEDs emit that light due to the compounds they use, as I mentioned earlier. Normal semiconductor diodes are made from silicon or germanium, but light-emitting diodes have compounds such as:

  • gallium arsenide
  • gallium arsenide phosphide
  • Silicium carbide
  • indium gallium nitride

Mixing these materials can produce a unique and different wavelength, in order to achieve the desired color. Different semiconductor compounds emit light in defined regions of the visible light spectrum and therefore produce different levels of light intensity. The choice of semiconductor material used in the manufacture of the LED will determine the wavelength of the photon emissions and the resulting color of the emitted light.

Radiation pattern

The radiation pattern is defined as the angle of light emission with respect to the emitting surface. The maximum amount of power, intensity or energy will be obtained in the direction perpendicular to the emitting surface. The light emission angle depends on the color being emitted and usually varies between about 80° and 110°. Here is a table with the different colors and materials:

Color
Wavelength (nm)
Voltage drop (V)
semiconductor materials
Infrared
> 760
<1,9
gallium arsenide
aluminum gallium arsenide
RED
610 – 760
1.6 – 2.0
aluminum gallium arsenide
gallium arsenide phosphide
aluminum gallium indium phosphide
gallium phosphide
Orange
590 – 610
2.0 – 2.1
gallium arsenide phosphide
aluminum gallium indium phosphide
gallium phosphide
Yellow
570 – 590
2.1 – 2.2
gallium arsenide phosphide
aluminum gallium indium phosphide
gallium phosphide
Verde
500 – 570
1.9 – 4.0
gallium indium phosphide
aluminum gallium indium phosphide
aluminum gallium phosphide
indium gallium nitride
BLUE
450 – 500
2.5 – 3.7
zinc selenide
indium gallium nitride
Silicium carbide
Silicon
Violeta
400 – 450
2.8 – 4.0
indium gallium nitride
Purple
multiple types
2.4 – 3.7
Dual blue/red LEDs*
Blue with Red Phosphorus
White with Purple Plastic
ultraviolet
<400
3.1 – 4.4
Diamond
boron nitride
aluminum nitride
aluminum gallium nitride
aluminum gallium indium nitride
Pink
multiple types
3.3
blue with phosphor
Yellow with red, orange or pink phosphor
White with pink pigment
Blanco
Spread spectrum
3.5
Blue/UV diode with yellow phosphor

The color of light emitted by an LED is not determined by the plastic body color that encloses the LED. This must be made very clear. As I mentioned earlier, epoxy resin is used both to improve light output and to indicate color when the LED is off.

In recent years, blue and white LEDs have also been developed, but they are more expensive than standard colored LEDs due to the production costs of mixing two or more complementary colors in an exact ratio within the semiconductor compound.

multi-color LEDs

On the market there is a wide variety of LEDs available, with different shapes, sizes, colors, output light intensities, etc. However, it must be said that the undisputed king for its price is the gallium arsenide phosphide red LED, with a diameter of 5mm. That is the most used in the world, so it is the one that is manufactured in the largest quantity.

However, as you have seen, there are currently many different colors, and several colors are even being combined to produce a multi-color LEDs like the one we are going to see in this section…

Bicolor

A bicolor LED, as its name suggests, is a LED capable of emitting in two different colors. This is achieved by combining two different colored LEDs in the same package. In this way, you can change from one color to another. For example, like those LEDs that you see on some devices to indicate the state of the battery charge that turn red when it is charging and green when it has already charged.

In order to build these LEDs are connected in parallel, with the anode of one LED connected to the cathode of another LED and vice versa. In this way, when power is supplied to any of the anodes, only one LED will light up, the one that is receiving power through its anode. If both anodes are powered at the same time, it is also possible to turn both on at the same time with dynamic switching.

Tricolor

We also have tricolor LEDs, that is, they can emit three different colors instead of two. These combine three LEDs with a common cathode in the same package, and to light one or two colors, you need to connect the cathode to ground. And the current supplied by the anode of the color you want to control or turn on.

That is, for one or two-color LED lighting, it is necessary to connect the power supply to either anode individually or at the same time. These tricolor LEDs are also often used in a multitude of devices, such as mobile phones, to indicate notifications, etc. Also, this type of diode generates additional shades of the primary colors by turning the two LEDs on at different ratios of direct current.

LED RGB

It is basically a type of tricolor LED, in this case known as RGB (Red Green Blue), because it emits those three colors lights. These have become very popular in colored trim strips and gaming gear, as you may know. However, even though you have the primary colors, it is not possible to generate all the colors and shades. Some colors fall outside the RGB triangle, and colors like pink, brown, etc. are hard to come by with RGB.

LED Advantages and Disadvantages

LEDs

Now it's time to see what are the main ones advantages and disadvantages of these LED diodes:

Advantages

  • Small size
  • low production cost
  • Long shelf life (will not melt)*
  • High energy efficiency / low consumption
  • Low temperature / less radiated heat
  • Design flexibility
  • They can produce many different colors, and even white light.
  • High switching speed
  • high light intensity
  • Can be designed to focus light in one direction
  • They are solid-state semiconductor devices, so they are more robust: more resistant to thermal shock and vibrations
  • No presence of UV rays
*Did you know that LED bulbs can be eternal. Sometimes they break and have to be replaced, but the truth is that the LED is still intact, what breaks is a capacitor that these bulbs have inside...

Disadvantages

  • Ambient temperature dependence of the radiant output power and the wavelength of the LED.
  • Sensitivity to damage due to excess voltage and/or excess current.
  • Theoretical overall efficiency is achieved only under special cold or pulsed conditions.

Applications

Bulb

Last but not least, it is necessary to show what are the possible applications for which these colored LEDs are intended:

  • for vehicle lights
  • Signage: indicators, signs, traffic lights
  • Display visual information on dashboards
  • For displays where the pixels are made up of LEDs
  • Medical applications
  • Toys
  • Lighting design
  • Remote controls (IR LEDs)
  • etc.

Be the first to comment

Leave a Comment

Your email address will not be published. Required fields are marked with *

*

*

  1. Responsible for the data: Miguel Ángel Gatón
  2. Purpose of the data: Control SPAM, comment management.
  3. Legitimation: Your consent
  4. Communication of the data: The data will not be communicated to third parties except by legal obligation.
  5. Data storage: Database hosted by Occentus Networks (EU)
  6. Rights: At any time you can limit, recover and delete your information.