Light Emitting Diodes

Introduction

A light emitting diode is a semi conductor device that gives out infrared or visible light after it has been charged with an electric current. It has a wide range of uses for example in the brake lights and rear windows of vehicles, indicator lamps, full color posters, bill boards and also alphanumeric displays. They are also used as a source of light in fiber optic telecommunication; in television remote controls and autofocus cameras. This paper seeks to discuss in detail the functioning of light emitting diodes.

History of Light Emitting Diodes

In 1907, a British engineer and inventor H.J Round was the first to report the emission of light when he passed current through a semi conductor rectifying junction. Round used a used a contact that was in between a silicon carbide crystal and a metal wire.  The John Allen’s group, at the Services Electronics Research Laboratory was the first to come up with first practical visible light emitting diode. The company set u a small production line in 1962. They used gallium phosphide that had controlled amounts of oxygen and zinc. Initially, this device was called “crystal lamps”, before it became known as “light emitting diode”. From 1962 into the late 1970’s, the devices made of zinc and oxygen dominated the LED market (Physics and Anatomy)

The General Electric Corporation was the first company to offer commercial Light Emitting Diodes. Their product gave out radiation in the red part of the spectrum. However, the company manufactured these products in low quantities as a result of the price of the product. A single light emitting diode went for 260 US Dollars. The LED was at that time offered through the Allied Radio Catalog which was widely distributed that dealt in amateur radio electronics.

The mass production of LED’s came about in 1968, by the Monsanto Corporation. The company established a factory that produced low cost LED’s. Monsanto then formed a collaboration of some sort with Hewlett Packard Corporation; it was expected that Monsanto would provide the raw materials whereas Hewlett Packard would produce the LED’s. This relationship however did not last long because Hewlett Packard felt uneasy about the idea of depending on a single source for its raw materials.

From the late 1960’s to the mid 1970’s, the greatest demand for LED’s came from calculators and then wrist watches; after the Hamilton Watch Corporation came up with the Pulsar digital watch. These two used numeric LED display. M. George Craford a manager and technical innovator at Monsanto, made huge contributions to the development of LED’s, with the most notable being the pioneering demonstration of a yellow LED (Shubert,F. pg 8-9).

The Technology of LED’s

In the light emitting diodes, the emission of light results when the positive hole and the negative electron, which are charge carriers, are injected into the semiconductor metallic contacts that are organic. These contacts are made on the opposite sides of the polymer films that are responsible for semi conducting. As soon as the hole and the electron come under the influence of the coulomb that they share, there is an attraction right inside the substance they recombine in thereby resulting in the emission of a photon. The emitted lights wavelength is dependent on the band gap of the semiconducting polymer. The holes and electrons are spin-1/2 particles, the resultant exciton can be a “single” exciton or a “triplet” exciton. Should the spin of the incoming charge carriers be random, then 75% triplet excitons and 25% singlet excitons will be formed. In a polymer material that has a relatively weak spin –orbit coupling, the only excitons that can recombine in a radiative manner are singlet excitons. This is because the ground state is a singlet state. LED is based on the principle of the injection luminescence principle.

The LED is constructed of gallium arsenide phosphide, gallium arsenide, gallium phosphide. Silicon and germanium are not suitable since p-n junction produce heat and hence no visible light.  The LED consists of p-n junction diode in which diffusion of potential is generated in the depletion layer between the n-type and p- type materials. The LED junction is connected in the forward biased mode. Electron move across the junction from n- type to p- type material thereby make the hole and the electron to recombine. The diffusion potential inhibits electrons and holes from leaving the n- and p regions since they enter opposite regions (Physics of Light Emitting Diodes). Applying the external forward biased voltage, V, the barrier is reduced to e(. If V the barrier becomes zero enabling electrons to flow from n- side to p-side. The electron injected into the depletion layer recombines with holes thereby emitting a photon energy which is given by the formula hv.

LED characteristics

During forward biased state, the current increases rapidly and hence there need to protect the LED to prevent it from getting destroyed. The light output is linearly proportional to the current within its active region.

LED can operate at low voltages and they don’t consume a lot of energy since all the energy is converted into light energy. They should not be operated at more than 40mA or 2.2V. The range of wavelengths that can be visible is from 0.4 µm to 0.7µm.

LED symbol

The symbol for an LED is as shown below.

Types of LED

There are four major types of LED and these are pinned LED, surface mounted LED, power LED and chip on board LED. Pinned LED is made from metal lead frame. Light emitting die is placed on the lead frame and molded around the frame forming LED. They can be diffused pinned LED that give a wide angle of light or water clear that give more directional beam. They are easy to use and easy to assemble.

The surface mounted LED has the smallest body into which the light emitting die is mounted. They are fitted into Printed circuit board. They are ideal at a premium space where they offer wide view angles of light.   The other type of LED is Power LED which gives high light output. They are mounted on heat conductive material (heat sink) that draws away heat from LED die to avoid damaging the LED. The chip on board type of LED uses a die bonded directly to printed circuit board or substrate that eliminates the body of an LED. The bonded die is covered in a clear protective resin which is hard to the die from being damaged. Its advantage is that it is less costly and they are small (LED lighting and signs).

New types of LED

Semiconductor technology has resulted into Innovation of new devices. These new device types are surface-emitting (large area) LED (SLED), micro-cavity surface emitter, edge-emitting LED (ELED)and super-luminescent diode emitter (SLD).Surface-emitting LEDs (SLEDs) are the conventional LEDs which emit light from relatively large surfaces oriented orthogonally to the axis of the emission pattern. Micro-cavity surface emitters are SLEDs with an internal mirror and layer thicknesses closely fitted to act as a low-finesse Fabry-Perot cavity and they do not show optical gain. The cavity is added to reduce the optical line width as well as the emission half-angle because a layer thickness forming the cavity reduces emission efficiency at larger angles

Edge-emitting LEDs have a device structure similar to that of the ridge wave guide laser but it does not have sufficient gain to lase. Typical dimensions of the emitting stripe are 3× 3 ×100 µm and it has active region hundreds of microns long. The energy density is concentrated in the long active region making it to achieve high radiances at the emitting facet thus; it becomes much easier to launch the light into an optical fiber. The emitting geometry for this device is different from the SLED. Super-luminescent edge emitters are similar to edge emitting LEDs, but they have wider central portion of the cavity than the emitting facet to allow more photons generated within the device reach the emitting facet than for a uniform cross-section, standard edge emitter, hence increases the emitted radiance (ICNIRP).

Considerations for use

LED specifications applicable to safety

 Radiance (brightness) is conserved and it cannot be increased by optical lensing and manufacturer’s specification sheet for an LED which is expressed as either radiance or luminance is not given. The radiant intensity is always specified. When the apparent source size is known, then LED radiance can be calculated. The actual source size is applicable in cases where no lens is incorporated, magnified and that apparent source size must be used all hazard assessment.

Exposure guidelines for eye safety

Occupational and public health exposure limits and guidelines state that LEDs should be treated as incoherent optical sources where different hazards are assessed severally over a range of wavelengths. Incoherent-source limits are more applicable to certain types of LEDs than others. Conventional, surface-emitting LEDs are radiance limited and many eye injuries have not been documented for LEDs. Laboratory efforts to create ocular injury with high-power LEDs have been unsuccessful while laboratory studies using diode lasers produced retinal injury.

Applications

LED Displays

Many businesses use LED lighting on their signs. Hey are also used in gas stations especially where the prices of gas are displayed, this gives the owner to change prices easily unlike in the old manual way.

LED Flashlights

Flashlights made of LED can be as little as key chains to high powered lanterns that are big in size. They are also used for traffic safety especially where roads are under construction where they are used to steer traffic away.

LED Lights for the Home

The use of LED lights is gaining popularity day after day especially for their use at home. It is increasingly being seen as a substitute to florescent lighting. They can be used as 121 volt DC light bulbs, flood lights, motion detecting lights among others. Led is also used in making toys and accessories such as glow sticks and it can also be used to light up jewelry for example necklaces, hair accessories, rings and bracelets. They are also used in making Christmas lights (Quinn,Lydia.).

Advantages and disadvantages of LEDs

Light emitting diodes are very efficient; they do not use a lot of electricity hence they are economical. The light emitted by an LED bulb is regarded to be a cool light therefore; one gets more light per watt as compared to an incandescent bulb. LED lights come in a wide range of sizes; they can be as small as 2mm or even less, unlike incandescent bulbs. They can therefore be used in instances where a regular bulb cannot be used.

These bulbs also come in a range of colors without necessarily requiring filters so as to produce these colors. LED can come in almost any color one could think of; the color depends on the material of the semiconductor and is therefore easier to come up with different colors. LED lights have the ability to achieve their optimum level of brightness in a few micro seconds, in other words, they have a more efficient on/off time.

Light emitting diodes last longer than incandescent bulbs. One can therefore get value for his money because of their lifetime. They also last longer and manage better for cycling; they don’t burn out that quickly and they are therefore economical (ledlights.org)

The main disadvantage of LED’s is their high cost. They are expensive on the basis of start up costs. These costs can be attributed to the low lumen output and power supplies that may be needed (ledlights.org)

Conclusion

Although the LEDs are expensive to use for the lighting applications because they are manufactured using the advanced semiconductor materials, they are the most cost effective lighting option in many situations. For instance, their long run lower costs make them better as compared to other light sources and they will play bigger role in the technology sector.