Yo, listen up here’s a story // About a little guy // That lives in a blue world
Blue his house // With a blue little window // And a blue corvette // And everything is blue for him
These verses correspond to “Blue (Da Ba Dee)”, a song by Italian music group Eiffel 65 that was first released in 1998 and became an international hit in 1999. It talks about a guy that lives in a reality where everything is blue. But would this guy have a blue LED lamp in his blue house? In this post we will answer this question and explain the importance of blue LEDs, whose inventors earned the Nobel prize in Physics in 2014.
A LED (light emitting diode) consists of a semiconductor chip in which a region has been positively doped (p-type region, impurities have been introduced to have additional positive charges, holes), and a region has been negatively doped (n-type region, with additional negative charges, electrons). When the required voltage is applied to this PN junction, the current generates electron-hole pairs that recombine producing the spontaneous emission of photons.
Depending on the semiconductors that are employed in the PN junction, the wavelength of the emitted light, that is, the colour we see, is different. The first LEDs that were manufactured were infrared and were based on germanium and silicon (1951, Bell Labs), and on gallium arsenide, GaAs (1961, Texas Instruments). The first red LED appeared in 1962 (Nick Holonyak Jr., General Electric) based on combining gallium phosphide (GaP) and GaAs. Finally, in the mid-1970s, pure GaP was employed to manufacture LEDs that emitted green light.
Once red and green LEDs had been obtained, blue was the only remaining primary colour to be obtained. In the late 1960s, several researchers (James Tietjen, Herbert Maruska, Jacques Pankove and Edward Miller) were working at RCA (Radio Corporation of America) to develop a blue LED based on gallium nitride (GaN). The main challenge was to obtain a p-doped GaN. Finally, the first blue LED based on GaN was created in 1972, although it was not very powerful and an alternative approach had been used as the problem of how to produce p-doped GaN still existed. Unfortunately, this research line was left due to budget problems.
More than a decade later, in the late 1980s, Japanese researchers Isamu Akasaki and Hiroshi Amano managed to create a p-type layer of GaN, and they introduced the first blue LED based on this layer in 1992. In parallel, engineer Shuji Nakamura was also able to create a p-type layer of GaN in 1992 using a different approach. During the 1990s, both teams developed different alloys of GaN employing aluminium or indium, creating more efficient blue LEDs. All three researchers also collaborated in the invention of the blue laser. They finally received the Nobel prize in Physics in 2014 for their invention of the blue LED.
But, why are blue LEDs so important? First of all, they are present in many devices that we use everyday, such as computers, mobile phones, cameras, LCD screens or laser printers… However, the one that is considered their most important application is “simply” to produce white light. There are not LEDs that directly produce white light. White light can only be obtained by combining red, blue and green light, or with a blue LED coated with a suitable phosphor that converts blue light to white light through fluorescence. In both cases, blue light is essential. Being able to produce white light with LEDs enables to replace incandescent light bulbs with this technology, thus greatly saving energy as LEDs are much more efficient. This fact was also considered in the 2014 Nobel Prize award.
In conclusion, blue LEDs are nowadays taken for granted, but it took decades to develop an efficient blue LED, and that was not so long ago. Returning to our initial question, our blue friend was probably able to enjoy a blue LED lamp in his blue house in 1998, as blue LEDs started to be mass-produced at the beginning of the 1990s. Today, however, our friend can not only enjoy blue LED light sources, but wavelengths ranging from the ultraviolet to the infrared, see more in Pyroistech’s catalogue of fiber coupled LED light sources.
Written by J.J. Imas