In this video, we will discuss Light Emitting Diode. So, the Light Emitting Diode is basically a P-N junction. The P-N junction made with materials that have a very high radiative recombination probability. So, here is a simple band diagram for a P-N junction. So, at equilibrium, this carrier diffusion is balanced by the electric field or the energy band bending built across the junction. When you apply a forward bias as shown in this figure in the right, then you lower the barrier, the carrier diffusion takes place. Now, if this is a long base diode, then as the carriers diffuse, the increased carrier concentration results in increased recombination rate, so the carriers will recombine. Now, if the material has a low radiative transition probability, recombination probability, then this recombination will probably be dominated by non-radiative processes such as shockly whole read recombination or oj recombination. But if your material has a very high radiative recombination probability, then the recombination could be mainly radiative and you get a lot of light coming out of your material, and this is essentially the Light Emitting Diode. So, the color or the spectrum of the light that you get would be related to the energies of the recombining electrons and holes, and if you look at the distribution of energy of the conduction band, the electrons and the valence band holes, there are basically congregated near the bottom of the conduction band and the top of the valence band and the lower energy site distribution of the conduction band electron is basically determined by the density of states. So, this goes up as square root of the energy and the high energy tail of the energy distribution of the conduction band electrons is determined mainly by the Fermi-Dirac distribution function. So, it decays exponentially with the energy on the high energy sites, and same thing with the hole distribution. So, the width of these electron energy distribution and the hole energy distribution is basically of the order of the thermal energy, kT, and therefore the width of the energy distribution of the resulting photons of the light is also of the order of kT, typically 2-3 kT and it has a sharp cutoff at the band gap energy. If you plot the spectrum as a function of wavelength, then of course the wavelength is inversely related to energy. So, this a spectrum is flipped and you have a sharp cutoff due to band gap at longer wavelengths side and you have an exponential tail on the shorter wavelength side, which is the high-energy site. So, the width of the spectrum for a red LED as shown here centered at 655 nanometer is about 24 nanometers and that's the again something of the order of kT, thermal energy at room temperature. If you plot the current versus the light intensity there basically linear because one electron and one hole produces one photon. So, they should be linearly proportional to each other and if you plot I-V characteristic, I versus V, then you get the standard exponential function dictated by your diode equation, ideal diode equation. So, here are some examples of semiconductor based LEDs, light emitting diodes emitting various different color. Gallium arsenide is one of the most efficient light emitting semiconductor and it has a band gap in the near infrared region around 900 nanometers and you can alloy Gallium arsenide with aluminum and increase the band gap and push the emission into the visible. So, there is a very efficient red LED based on Aluminum gallium arsenide. Indium gallium arsenic phosphide is an IR emitting material and this is a material that is commonly used for communications, infrared fiber based communications, and then of course there is this nitride, Gallium nitride based material which emits in the blue and green region. Now, I want to have a few more words about nitride based LEDs because historically, the blue LED was the most challenging to develop. Now, LED is an excellent light source. It has a very high efficiency and it has a very fast response time and it is a solid state device, so it's very stable has a long life and very low failure rate compared to other light sources for example, as a fluorescent tube or light bulb, the incandescent light bulb. So, there are many potential applications and displays, there are many displays that are currently using LEDs as light source, TVs and other things. Also, there are many solid state lighting lamps, the general illumination lamps made of LEDs. Now, in order to make this you need to produce full colour, meaning that you need a color source, light source that are emitting red, green and blue colors. Aluminum gallium arsenide LED has been around for a long time. Gallium phosphide LED produces green, gallium phosphide is an indirect band gap material so efficiency is not very high but it was available. Blue light emitting material was just not available. So, there has been a lot of research and development effort to develop blue LED and laser diode and gallium nitride is now the dominant material system that produces this. In fact, the developers of Gallium nitride based LED has received the Nobel Prize in physics some years ago for their contribution to these solid state lighting devices. The typical white LED or solid state lighting device is composed of blue emitting LED Gallium nitride based, Indium gallium nitride to be exact and then you coated with a phosphor, a luminescent material, photo luminescent material that absorbs part of this blue light coming out of your LED and converted into yellow. This blue light leaking through plus the yellow light converted by the phosphor combined produces white light. So, generally this is a cheap low grade lamp and you could produce a better white color that mimics the sunlight more faithfully by using a ultraviolet LED, again gallium nitride based. You have a mixture of RGB phosphors excited by this UV light. So, this is basically the exact same mechanism as the fluorescent lamps. Fluorescent lamp uses a mercury discharge that produces UV and the phosphor materials coded on the lamp wall then converts the UV into RGB visible light to produce white. So, this architecture basically eliminates the mercury source and replace it with a reliable and high efficiency solid state source made of semiconductor. You could also have a multi chip, multi semiconductor based white light by stacking up these red, green and blue LEDs. So, in this case there are no phosphors and all of those visible primary colors RGB are produced by semiconductor based LED devices. Both of these two guys can give you a very high quality white light and can make a high quality lamps and also a full color display.
