According to Nobel committee citation, this year’s Nobel Prize for Physics has been awarded to Isamu Akasaki and Hiroshi Amano, Nagoya University, Japan, and Shuji Nakamura, University of California, Santa Barbara, for the invention of efficient blue-light emitting diodes (LEDs). These have enabled bright and energy-saving white-light sources. In the spirit of Alfred Nobel, the prize rewards an invention of greatest benefit to mankind, and by using blue LEDs, white light can be created in a new way.
With the advent of LED lamps, we now have more long-lasting and efficient alternatives to older light sources. The committee added, “The LED lamp holds great promise for increasing the quality of life for over 1.5 billion people around the world who lack access to electricity grids. Due to low-power requirements, it can be powered using cheap, local solar power.”
The blue LED invention has made possible a new energy-efficient and environment-friendly light source. There are many different kinds of LEDs and most common types are categorised by colour, viewing angle, lens type, dimensions, forward voltage, forward current, packaging type and intensity.
Despite considerable efforts, both in the scientific community and in industry, the blue LED had remained a challenge for three decades. When these scientists produced bright-blue-light beams from their semiconductors in the early 1990s, they triggered a fundamental transformation of lighting technology. Red and green diodes had been around for a long time but, without blue light, white lamps could not be created. They succeeded where everyone else had failed.
The invention of blue LED is just twenty years old but, it has already contributed to create white light in an entirely new manner to benefit us all. White LED lamps emit a bright-white light, are long-lasting and energy-efficient. These are being constantly improved upon, getting more efficient with higher luminous flux (measured in lumen) per unit electrical input power (measured in watt).
The most recent record is just over 300-lm/W, which can be compared to 16-lm/W for regular light bulbs and close to 70-lm/W for fluorescent lamps. As about one fourth of the world’s electricity consumption is used for lighting purposes, LEDs will contribute to saving Earth’s resources. Materials consumption is also diminished as LEDs last up to 100,000 hours, as compared to 1000 hours for incandescent bulbs and 10,000 hours for fluorescent lights.
Electronics of LED
In the case of LEDs, the conductor material is typically aluminium-gallium-arsenide (AlGaAs). In pure AlGaAs, all atoms bond perfectly to their neighbours, leaving no free electrons (negatively-charged particles) to conduct electric current. In doped material, additional atoms change the balance by either adding free electrons or creating holes where electrons can go. Either of these alterations makes the material more conductive.
A semiconductor with extra electrons is called n-type material, since it has extra negatively-charged particles. In n-type material, free electrons move from a negatively-charged area to a positively-charged area.
A semiconductor with extra holes is called p-type material, since it effectively has extra positively-charged particles. Electrons can jump from hole to hole, moving from a negatively-charged area to a positively-charged area. As a result, holes themselves appear to move from a positively-charged area to a negatively-charged area.
A diode consists of a section of n-type material bonded to a section of p-type material, with electrodes on each end. This arrangement conducts electricity in only one direction. When no voltage is applied to the diode, electrons from the n-type material fill holes from the p-type material along the junction between the layers, forming a depletion zone.
In a depletion zone, the semiconductor material is returned to its original insulating state—all holes are filled, so there are no free electrons or empty spaces for electrons, and charge cannot flow.
To get rid of the depletion zone, you have to get electrons moving from the n-type area to the p-type area and holes moving in the reverse direction. To do this, you connect the n-type side of the diode to the negative end of a circuit and the p-type side to the positive end. The free electrons in the n-type material are repelled by the negative electrode and drawn to the positive electrode. The holes in the p-type material move the other way. When the voltage difference between electrodes is high enough, electrons in the depletion zone are boosted out of their holes and begin moving freely again. The depletion zone disappears and charge moves across the diode.
In an electronics circuit, an LED behaves very much like any other diode. LEDs are often used to indicate the presence of a voltage at a particular point and are often used as a supply-rail indicator.
When used in this fashion, there must be a current-limiting resistor placed in the circuit. The resistor’s value should be calculated to give the required level of current. For many devices, a current of around 20mA is suitable, although it is often possible to run them at a lower current. If less current is drawn, the device will obviously be dimmer.
When calculating the amount of current drawn, voltage across the LED itself may need to be taken into consideration. The voltage across an LED in its forward-biased condition is just over a volt, although the exact voltage is dependent upon the diode and, in particular, its colour. Typically, a red one has a forward-voltage of just below two volts, and it is around 2.5 volts for green or yellow.
Great care must be taken not to allow a reverse bias to be applied to the diode. Usually, LEDs have a reverse breakdown of very few volts. If breakdown occurs, then the LED is destroyed. To prevent this from happening, an ordinary silicon diode can be placed across the LED in reverse direction to prevent any reverse bias being applied.
Development of LEDs
LED, in electronics, is a semiconductor device that emits infra-red or visible light when charged with an electric current. LEDs operate by electro-luminescence, a phenomenon in which emission of photons is caused by electronic excitation of a material. The material used most often in LEDs is gallium-arsenide, though there are many variations on this basic compound, such as AlGaAs or aluminium-gallium-indium-phosphide.
These compounds are members of the so-called III-V group of semiconductors, that is, compounds made of elements listed in columns III and V of the periodic table. By varying the precise composition of the semiconductor, the wavelength (and therefore the colour) of the emitted light can be changed.
LED emission is generally in the visible part of the spectrum (with wavelengths from 0.4 to 0.7 micrometre) or in the near infra-red (with wavelengths between 0.7 and 2.0 micrometres). The brightness of the light observed from an LED depends on the power emitted by the LED and on the relative sensitivity of the eye at the emitted wavelength. Maximum sensitivity occurs at 0.555 micrometre, which is in the yellow-orange and green region. The applied voltage in most LEDs is quite low, in the region of two volts. The current depends on the application and ranges from a few milliamperes to several-hundred milliamperes.
The term diode refers to the twin-terminal structure of an LED. In a flashlight, for example, a wire filament is connected to a battery through two terminals—one (the anode) bearing the negative electric charge and the other (the cathode) bearing the positive charge. In LEDs, as in other semiconductor devices such as transistors, the terminals are actually two semiconductor materials of different composition and electronic properties brought together to form a junction.
In one material (the negative, or n-type semiconductor), the charge carriers are electrons, and in the other (the positive, or p-type semiconductor), the charge carriers are holes created by the absence of electrons. Under the influence of an electric field (supplied by a battery, for instance, when the LED is switched on), current can be made to flow across p-n junction, providing the electronic excitation that causes the material to luminance.
In a typical LED structure, the clear epoxy dome serves as a structural element to hold the lead frame together, as a lens to focus the light and as a refractive-index match to permit more light to escape from the LED chip. The chip, typically 250×250×250 micrometres in dimension, is mounted in a reflecting cup formed in the lead frame.
The p-n-type GaP:N layers represent nitrogen added to gallium-phosphide to give green emission, the p-n-type GaAsP:N layers represent nitrogen added to gallium-arsenide-phosphide to give orange and yellow emission, and the p-type GaP:Zn,O layer represents zinc and oxygen added to gallium-phosphide to give red emission.
Two further enhancements, developed in the 1990s, are LEDs based on aluminium-gallium-indium-phosphide, which emit light efficiently from green to red-orange, and also blue LEDs based on silicon-carbide or gallium-nitride. Blue LEDs can be combined on a cluster with other LEDs to give all colours, including white, for full-colour moving displays.
If we try to run current the other way, with the p-type side connected to the negative end of the circuit and the n-type side connected to the positive end, current will not flow. The negative electrons in the n-type material are attracted to the positive electrode. The positive holes in the p-type material are attracted to the negative electrode. No current flows across the junction because the holes and the electrons are each moving in the wrong direction. The depletion zone increases. The interaction between electrons and holes in this setup has an interesting side effect—it generates light.
Light is a form of energy that can be released by an atom. It is made up of many small, particle-like packets that have energy and momentum but no mass. These particles, called photons, are the most basic units of light. Photons are released as a result of moving electrons. In an atom, electrons move in orbitals around the nucleus. Electrons in different orbitals have different amounts of energy. Generally speaking, electrons with greater energy move in orbitals farther away from the nucleus. For an electron to jump from a lower orbital to a higher orbital, something has to boost its energy level.
Conversely, an electron releases energy when it drops from a higher orbital to a lower one. This energy is released in the form of a photon. A greater energy-drop releases a higher-energy photon, which is characterised by a higher frequency.
In case of LEDs, free electrons moving across a diode can fall into empty holes from the p-type layer. This involves a drop from the conduction band to a lower orbital, so electrons release energy in the form of photons. This happens in any diode, but you can only see the photons when the diode is made of certain material.
Atoms in a standard silicon diode, for example, are arranged in such a way that the electron drops a relatively shorter distance. As a result, the photon’s frequency is so low that it is invisible to the human eye—it is in the infra-red portion of the light spectrum.
This is not necessarily a bad thing. Infra-red LEDs are ideal for remote controls, among other things. Visible light emitting diodes (VLEDs), such as the ones that light up numbers in a digital clock, are made of materials characterised by a wider gap between the conduction band and lower orbitals. The size of the gap determines the frequency of the photon—it determines the colour of the light.
While LEDs are used in everything from remote controls to digital displays on electronics, VLEDs are growing in popularity and use, thanks to their long lifetimes and miniature size. Depending on the materials used in LEDs, these can be built to shine in infra-red, ultra-violet and all colours of the visible spectrum in between.
When current flows across a diode, negative electrons move in one way and the positive holes move in another way. The holes exist at a lower-energy level than the free electrons, so when a free electron falls, it loses energy. This energy is emitted in the form of a light photon. The bigger fall of electron means higher energy or frequency of light emitted.
Light emitting diodes are real unsung heroes of the electronics world. These do dozens of different jobs and are found in all kinds of devices. Among other things, these form numbers on digital clocks, transmit information from remote controls, light-up watches and tell you when your appliances are turned-on. Collected together, LEDs can form images on a jumbo television screen or illuminate a traffic light.
Visible LEDs are used in many electronic devices as indicator lamps, in automobiles as rear-window and brake lights, and on billboards and signs as alpha-numeric displays or even full-colour posters.
Infra-red LEDs are employed in autofocus cameras and television remote controls, and also as light sources in fibre-optic telecommunication systems.
Any LED can be used as a light source for a short-range fibre-optic transmission system, that is, over a distance of less than 100 metres (330 feet). For long-range fibre optics, however, the emission properties of the light source are selected to match the transmission properties of the optical fibre, and in this case, infra-red LEDs are a better match than visible LEDs.
Glass optical fibres suffer from lowest transmission losses in the infra-red region at wavelengths of 1.3 to 1.55 micrometres. To match these transmission properties, LEDs are employed that are made of gallium-indium-arsenide-phosphide, layered on a substrate of indium-phosphide. The exact composition of the material may be adjusted to emit energy precisely at 1.3 or 1.55 micrometres.
Basically, LEDs are just tiny light bulbs that fit easily into an electrical circuit. But unlike ordinary incandescent bulbs, these do not have a filament that will burn out, and these do not get especially hot. LEDs are illuminated solely by the movement of electrons in a semiconductor material, and these last just as long as a standard transistor.
The interior of an LED is actually quite simple, which is one of the reasons this technology is so versatile. The lifespan of an LED surpasses the short life of an incandescent bulb by thousands of hours. LEDs also fit more easily into modern electronic circuits.
The main advantage is efficiency as LEDs generate very little heat. A much higher percentage of the electrical power goes directly to generating light, which cuts down on the electricity demands considerably. Tiny LEDs are already replacing tubes that light-up LCD HDTVs to make dramatically-thinner televisions. Though these often come in tiny packages, LEDs produce a large amount of light and are used in an ever-growing list of technologies.
Although LEDs will continue to be widely used as small indicator lamps, the number of applications these can find is increasing as the technology improves. New very-high luminance diodes are now available. LEDs are even being used as a form of illumination, an application which these were previously not able to fulfil because of their low-light output. New colours are being introduced. White and blue LEDs, which were previously very difficult to manufacture, are now available. In view of the on-going technological development, and their convenience of use, these devices will remain in electronics catalogues for years to come.
While all diodes release light, most do not do it very effectively. In an ordinary diode, the semiconductor material itself ends up absorbing a lot of the light energy. LEDs are specially constructed to release a large number of photons outward. Most of the light from the diode bounces off the sides of the bulb, travelling on through the rounded end.
Additionally, LEDs are housed in a plastic bulb that concentrates the light in a particular direction. Up until recently, LEDs were too expensive to use for most lighting applications because these are built around advanced semiconductor material. But advantages like energy-efficiency and longer lifespans have made LEDs a more cost-effective lighting option for a wide range of situations, competing with incandescent and compact fluorescents.