- summary of light emitting diode, LED history, detailing how it was discovered and finally developed into a product that is now being widely used.
LED tutorial includes:
• LED types • LED technology • LED history • LED characteristics • LED specifications & parameters • LED configurations & packages • LED structure & fabrication • OLED basics • OLED technology & operation • PMOLED - Passive Matrix OLED • AMOLED - Active Matrix OLED • High Brightness, HBLED • LED life expectancy
LED history dates back many more years than many people imagine. LED history dates back to the beginnings of radio and electronics.
While LEDs have now been available since the 1960s, the LED history extends many years before this. The LED took many years to develop for a number of reasons - the first discoveries were well ahead of their time, other discoveries were lost. It was only when the technology was sufficiently mature that the LED could be fully developed and marketed. Even after the first devices appeared, LED history was not finished - new developments have been made and LEDs are addressing new markets, never really envisaged before.
Early LED history
The first recorded effects of the light emitting diode effect were noticed back at the beginning of the twentieth century. A British engineer named H J Round working for Marconi was undertaking some experiments using crystal detectors. At the time radio detectors were one of the major limiting factors within the early wireless of radio sets.
The early detectors were often made by using a small piece of material - we would now know them as forms of semiconductor - and placing a small wire onto the surface. These were called "Cat's Whiskers" for obvious reasons. In trying to investigate the effects and improve their performance, Round had passed a current through some of his detectors. He noted that one of them emitted light when a current was passed through it. Although he did not understand the mechanism for the effect, he published his findings in 1907 in a magazine of the day named Electrical World.
LEDs investigated by Losov
The idea lay dormant for some years before it was observed again by a Russian engineer named Oleg Vladimirovich Losov. He was the son of a Russian Imperial Army Office - born into a noble family. This would have counted against him in the post revolution Russia era.
Losov has attended a number of university lectures but never undertook any formal university education, but instead was a technical at the Leningrad Medical Institute.
Losov made some major advances and is a key person in the LED history. He undertook a considerable amount of work investigating light emission from Cat's Whisker style detectors. He observed and investigated the light emission from zinc oxide and silicon carbide crystal rectifiers.
As a result of his observations and investigations, Losov published a number of papers in the technical press of the day between 1924 and 1930. His first paper was entitled: "Luminous carborundum detector and detection crystals" which was published in a Russian journal. Soon he published his findings in other British and German. Losov detailed a variety of aspects of these diodes including the spectra of their light emission as well as many other aspects of their operation. In one article published in the Philosophical Magazine in 1928 he detailed the I-V characteristics of a carborundum diode along with the onset of light emission. This formed part of his work on investigating the nature of the diode emission - recognising it was not a thermal effect, but arising from the semiconductor action.
In further work, Losov investigated the temperature relationships of the effect, cooling the semiconductors down to very low temperatures. He also modulated the LED to see the effects of frequency of any current applied to the diode.
Losov went on to investigate further ideas associated with diode and what would alter be called semiconductor technology. Sadly though, he lived in Leningrad and he was killed during the siege of Leningrad during the Second World War. He had published a total of four patents between 1927 and 1942, but all this work was lost as records were destroyed in Leningrad.
Semiconductor technology advances
During the Second World War, radar was seen as a major enabler. Accordingly a large amount of development of practical devices was initiated. This utilised much of the materials science work that had been undertaken in the 1920s and 1930s.
As a result of the work new point contact diodes were developed. These were able to provide better performance than thermionc valve / vacuum tube diodes. As a result of the research into semiconductor diodes, the idea for the light emitting diode re-surfaced in 1951. This time work was to be more successful, although it took some years to reach completion. One research team was lead by Kurt Lehovec. He applied for a patent in 1952 for Silicon carbide diodes that emitted light. However this was only the first phase of the work that was needed.
Following the work by Kurt Lehovec, others also started to work on LED technology. The work took many years and involved a number of companies and researchers. Even Shockley became involved.
Although LEDs did not become commercially available for a number of years, several people made some significant discoveries and improvements. Lehovac himself investigated introducing different impurities to change the colour of the light making blue, green/yellow, and pale yellow from different combinations.
Also researchers working at RCA patented a green LED in 1958. All of these LED developments added more to LED technology, furthering the technology within the overall LED history.
Commercial LED history
The first commercially available LEDs started to appear in the late/mid 1960s. These LEDs early LEDs used a semiconductor made using gallium, arsenic and phosphorus - GaAsP. This produced a red light, and although the efficiency of the devices was low (typically around 1 - 10 mcd at 20mA) they started to be widely used as indicators on equipment.
One of the first companies to manufacture LEDs on any scale was Monsanto. Monsanto was actually a company supplying the raw semiconductor materials. They had aimed at working with Hewlett Packard - then a test equipment company - with Monsanto supplying the semiconductor and Hewlett Packard manufacturing the diodes. However the relationship did not work out and Monsanto ended up developing the LEDs themselves. [The name of Monsanto is not seen today. The business was sold in 1979 to General Instrument.]
With the original GaAsP devices being manufactured, the next development saw gallium phosphide devices developed. GaP devices were not widely used because the light they produced was at the far end of the red spectrum where the sensitivity of the human eye is low, and even though they produced a high output, the human perception was of a dim light.
High output LED lamps
As LEDs were developed, the light levels increased to the extent that they could be considered for applications outside simple indicator lamps. By 1987 the Hewlett Packard AlGaAs (aluminium gallium arsenide) diodes being produced were bright enough for the first applications within lighting. The first applications for these diodes was within the automotive industry where red LEDs were used for vehicle brake lights, and also for traffic lights. Here the use of LEDs was of particular interest because of their increased reliability over the incandescent lights that had been previously used.
A year after the first AlGaAs LEDs were introduced another variant, AlInGaP (aluminium Indium Gallium Phosphide) were manufactured. These LEDs gave a significant improvement over the previous AlGaAs diodes by doubling the light output.
Later, in 1993 HP started to use GaP (gallium phosphide) to provide high output green LEDs. Also further developments of this technology allowed the production of high output orange lamps. These were ideal for use as car direction indicators - again their reliability in being turned on and off as well as their efficiency proved to be a major improvement.
By Ian Poole
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