Superbright LED is an advertising ploy, an epithet on which sellers lure unsuspecting customers. In fact, it relies solely on efficiency to pay attention.
The concept of brightness
Few studies of the characteristics of the LED in this issue, the limitations imposed by human physiology. The sensitivity of the eye to the waves of green is an order of magnitude higher than the analogous parameter for red. It is not enough to calculate the power flux-density, it is not enough to make sure that the thermal regime does not go beyond what is permitted, thanks to good efficiency. It is required to impose the resulting result on the features of human vision.
Now it becomes clear that the statements of manufacturers about super-bright LEDs are only an advertising gimmick. It is supposed to evaluate the product in the complex, but even then remember - dear reader - that coherent light is dangerous for the eye. You should not check the products on their own vision.
It is already painful to look at an ordinary 10-watt LED light when the radiating matrix shines through a frosted glass. The authors are confident that it is permissible to call any presented super-bright LED.
The history of the development of
Most diodes operate due to the luminescence effect, discovered at the beginning of the 20th century. It is believed that the first LEDs were made unintentionally by Henry Joseph Round when he evaluated the rectifying properties of silicon carbide. It is noteworthy that the mineral carborundum on planet Earth is almost never found, although it is extremely common in stellar atmospheres.
From there a meteorite arrived, which was too tough for Eugene Achison in 1891.The idea of the excavator is quite understandable - he decided that he had discovered diamonds on the dead asteroid and wanted to quietly sell the find. But the jeweler noted that there are no characteristic signs of the most precious stone on the planet. And it happened years later.
Henry Joseph Round Carborund was artificial. At the beginning of the 20th century, the mineral was already learned to synthesize. In hardness the stone is inferior only to diamond. Investigating a crystal detector for radio( encouraged by the experience of other researchers who have already patented), Henry discovered a glow. He immediately wrote to the editorial board of the magazine Electric World and reported this information:
- At a voltage of 10 V AC, samples of carborundum in yellow light up.
- As the potential difference increases up to the 110-volt mains, all experimental crystals show luminescence.
- As the voltage in the spectrum increases, in addition to yellow, green, orange and blue colors are noted.
- Individual materials glow only from the edge, others show a volumetric effect.
- The phenomenon is not explained by thermoelectricity.
Glow occurs when the pn junction is biased. With a large applied voltage, a considerable number of minority charge carriers penetrate into the crystal. The process is explained by the tunnel effect. When the "guest tour" begin to recombine with the main charge carriers, the excess energy turns into light. This explains the fact that, at low voltages, Henry Joseph did not observe Round.
However, not everything is so simple. Schottky diodes — represented by carborundum with metal contacts — can also glow with a negative applied voltage. The scheme is exactly the same, but with a significant potential difference, an avalanche breakdown occurs. The semiconductor atoms are ionized by accelerated charge carriers, the reverse recombination is performed with the emission of a photon of light.
Warning! Modern LEDs emit only at a direct displacement of the pn-junction, when a positive potential is applied to the anode.
The works of the Round were repeated by Russian Losev in 1928.The scientist on the crystal detector managed to get a glow and found that the first samples glow only with a unipolar connection, and for others the direction of direct current does not matter. Attempts to comprehend the fact did not lead to the result. But the conclusion of the Round was confirmed that the effect is not associated with thermoelectric heating.
The beginning of the LED era is considered the early 60s, when the first carborundum films appeared. The efficiency of the first specimens was amazingly small and amounted to 0.005%.The reason is simple - silicon carbide is far from the best material for the manufacture of super-bright diodes. The latter is not feasible at this stage of technology.
Which is better?
In the early 90s, carborundum disappeared from the shelves. The last blue LEDs emitted in the range of 470 nm with an efficiency of 0.03%.
Already in the 50s, semiconductors from the AIIBVI group were well studied. Produced a constant search for new technical solutions. The light-emitting diodes from III-V class semiconductors appeared, using the example of which teachers of physics explain impurity conductivity. Materials of this type of artificial origin are not found in nature. By doping gallium with arsenic, scientists obtained a new field for research. Impurities were injected onto the substrate by liquid-phase or gas-phase epitaxy.
By 1962, lasers had already appeared on the basis of the material described. They were predicted a great future in the space industry, suitable for communication and measurements. The serial production of LEDs based on gallium arsenide was undertaken by Texas Instruments. The price of the piece was 130 dollars. Today, the cost of LEDs has greatly decreased, and gallium arsenide is massively used to create control panels, communication devices and other things.
Phosphorylated gallium arsenide.
The efficiency of known materials turned out to be too small to create super-bright LEDs. So Holonyak and Bevac came in 1962 to the need for phosphorylation of gallium arsenide to improve performance. A feature of the new devices was the high coherence of radiation. This meant that communications equipment was waiting for further improvements, beam homogeneity plays a big role.
Before it was about the development mainly of IBM engineers, except for the secret NASA projects. In 1962, the famous General Electric joined the struggle. Growing crystals by gas-phase epitaxy, the engineers of the company have achieved notable success. The efficiency of the devices was quickly increased, but the coherence of the radiation was greatly reduced. The price of General Electric was twice as high as Texas Instruments, the batch came out scanty.
In 1968, Monsanto bought the rights and started mass production of LEDs based on phosphorylated gallium arsenide. Sales grew annually at least fourfold, but remained absolutely microscopic in absolute terms. Finally, the first LED digital displays appear.
In parallel, the technology of gallium phosphide production developed. Each firm of the industry struggled with its own unique material. Gallium phosphide was taken up by Bell Laboratories. This was probably not a deliberate strategy, firms were afraid of mutual absorption. Although the fact of uniformity is alarming.
Gallium phosphide LEDs made it possible to get a yellow and red glow. Bell Labs began working together with others in the early 60s. What makes you think about the planned action. The first publications were independent and made only by two scientists( 1964):
Tin-alloyed LED transitions from gallium phosphide are named after them. The data obtained that the optical properties are greatly improved by the introduction of impurities of nitrogen. Annealing the structure of a semiconductor after its growth, the efficiency was able to increase to 2%.At the same time, a search was made for new color qualities. So created diodes based on gallium phosphide, giving a green tint, the efficiency was 0.6%.
However! The efficiency of green LEDs is lower, but because of the increased susceptibility of the eye to the green range, they seemed brighter than red.
efficiency In order for the LED to become super-bright, it is characterized by high efficiency. The logic is elementary. The higher the current, the greater the loss on the ohmic resistance of the contacts. Consequently, to obtain high brightness with low efficiency, the current is extremely increased. The semiconductor will not stand and melt. It was not for nothing that the first laser worked with cooling to 77 K. In addition to its physical qualities, this ensured proper cooling.
An ideal LED with an efficiency of 100% radiating one photon for each electron injected. This is called a quantum yield, ideally equal to one. In a real LED, the efficiency is estimated by the ratio of the power of optical radiation to the injection current.
The emitted photons should go into space. For this, if possible, the area of the pn-junction opens. In reality, a significant part of the photons remains inside. Therefore, each design, among other things, is characterized by an optical output. Typically, the parameter becomes the main limiting factor, barely reaching 50%.
The efficiency of an LED is commonly understood as the ratio of the number of emitted photons to the summed up power. Typically, a voltage on the p-n junction is on the order of one and a half volts, and then the current rises linearly. Consequently, the power is lost to the displacement of the barrier layer, the radiation and the heating of the ohmic resistance. At the beginning of the XXI century, the LED efficiency of 4% was considered normal( taking into account the optical output).
In order to increase the output and finally get a super-bright LED, engineers began to look for new constructive solutions.
Improving the efficiency of LEDs
Increasing the luminosity of the diode is achieved by maintaining a high concentration of carriers. The method of achievement is the creation of a double pn-junction. In this case, the radiation layer is surrounded by semiconductors of a different type of conductivity on both sides, increasing the area of casting of minority carriers. The design looks like a 5-layer sandwich:
- The active radiation layer is in the center.
- On both sides it is covered by semiconductors, which causes the presence of two locking layers.
- Contacts cover outer semiconductors over the entire area to improve current flow.
The quantum yield depends on the core thickness. The graph is non-linear and demonstrates a pronounced flat or sloping hump. Accordingly, the thickness value is required to choose from its limits, which are tens of microns. Experiments show that increases in the quantum yield are achieved by weak doping of the active region. The number of impurity atoms does not exceed ten to the seventeenth power of units per cubic centimeter. In general, the process is relatively poorly understood.
Increased injection is achieved by doping extreme layers. The concentration of impurities here is at least an order of magnitude lower than in the previous case, or a similar number of times higher. Although the barrier and active layers are by definition represented by different materials, it is important that their crystal lattices be identical in structure. With increasing mismatch, the quantum yield drops sharply.